diff --git a/__tests__/component/Viewer.spec.ts b/__tests__/component/Viewer.spec.ts
index aaa6fc9..310e9fe 100644
--- a/__tests__/component/Viewer.spec.ts
+++ b/__tests__/component/Viewer.spec.ts
@@ -46,6 +46,17 @@ describe('Viewer.open()', () => {
 		expect(modalProps().show).toBe(true)
 	})
 
+	it('titles the modal with the display name when the server gives one', async () => {
+		const { vm, wrapper, modalName } = mountViewer([imageHandler()])
+		// A version of a file is served under its version id but reads as a date
+		const version = makeFile({ basename: '1737542400', mime: 'image/jpeg', displayname: '22 January 2025, 11:20:00' })
+
+		await vm.open([version], version)
+		await wrapper.vm.$nextTick()
+
+		expect(modalName()).toBe('22 January 2025, 11:20:00')
+	})
+
 	it('filters currentFileList to files of the same handler group', async () => {
 		const pdfHandler = makeHandler({
 			id: 'pdf',
@@ -305,6 +316,30 @@ describe('Viewer action submenu', () => {
 	})
 })
 
+describe('Viewer sidebar', () => {
+	const sidebarButton = (wrapper: ReturnType<typeof mountViewer>['wrapper']) => wrapper.findAll('.nc-action-button-stub').find((button) => button.text().includes('Open sidebar'))
+
+	it('offers the sidebar for an ordinary file', async () => {
+		const { vm, wrapper } = mountViewer([imageHandler()])
+		const f1 = makeFile({ mime: 'image/jpeg' })
+
+		await vm.open([f1], f1)
+		await wrapper.vm.$nextTick()
+
+		expect(sidebarButton(wrapper)).toBeTruthy()
+	})
+
+	it('does not offer it for a file the sidebar cannot resolve', async () => {
+		const { vm, wrapper } = mountViewer([imageHandler()])
+		const version = makeFile({ mime: 'image/jpeg' })
+
+		await vm.open([version], version, { enableSidebar: false })
+		await wrapper.vm.$nextTick()
+
+		expect(sidebarButton(wrapper)).toBeUndefined()
+	})
+})
+
 describe('Viewer loadMore', () => {
 	it('appends files returned by loadMore when reaching the last item', async () => {
 		const f1 = makeFile({ basename: 'f1.jpg', mime: 'image/jpeg' })
diff --git a/__tests__/defaults.spec.ts b/__tests__/defaults.spec.ts
index 94cbba2..542ca7e 100644
--- a/__tests__/defaults.spec.ts
+++ b/__tests__/defaults.spec.ts
@@ -20,6 +20,18 @@ describe('default handlers', () => {
 		expect([...scope.handlers!.keys()].sort()).toEqual(['audios', 'images', 'videos'])
 	})
 
+	it('can be reached by importing one of the handler modules first', async () => {
+		// Entering the graph anywhere but the entry used to hit the entry
+		// mid-evaluation, and the handler it was about to register was not
+		// initialised yet
+		vi.resetModules()
+		const { registerImageHandler } = await import('../lib/models/images.ts')
+
+		registerImageHandler()
+
+		expect(scope.handlers!.has('images')).toBe(true)
+	})
+
 	it('do not complain about themselves when asked for explicitly', async () => {
 		const { registerDefaultHandlers } = await importPackage()
 		const { logger } = await import('../lib/services/logger.ts')
diff --git a/__tests__/factories.ts b/__tests__/factories.ts
index f0b11db..adba976 100644
--- a/__tests__/factories.ts
+++ b/__tests__/factories.ts
@@ -19,6 +19,8 @@ interface MakeFileOptions {
 	mtime?: Date
 	size?: number
 	root?: string
+	/** What the server calls the file, when that differs from its name */
+	displayname?: string
 }
 
 /**
@@ -41,6 +43,7 @@ export function makeFile(options: MakeFileOptions = {}): File {
 		// A dav node reports whatever it was given, and a file nobody can
 		// read is not what these tests are about unless they say so
 		permissions: options.permissions ?? Permission.ALL,
+		displayname: options.displayname,
 	})
 }
 
diff --git a/lib/components/ImageEditor.vue b/lib/components/ImageEditor.vue
index 529ba04..ed72568 100644
--- a/lib/components/ImageEditor.vue
+++ b/lib/components/ImageEditor.vue
@@ -5,7 +5,7 @@
 <template>
 	<NcImageEditor
 		:src="file.source"
-		:label="file.basename"
+		:label="file.displayname"
 		:exportOptions="exportOptions"
 		:saving="saving"
 		class="viewer__image-editor"
diff --git a/lib/components/Images.vue b/lib/components/Images.vue
index e465118..df9cc07 100644
--- a/lib/components/Images.vue
+++ b/lib/components/Images.vue
@@ -131,7 +131,7 @@ const previewPath = computed(() => getPreviewIfAny(props.file))
 
 const zoomHeight = computed(() => Math.round(height.value * zoomRatio.value))
 const zoomWidth = computed(() => Math.round(width.value * zoomRatio.value))
-const alt = computed(() => props.file.basename)
+const alt = computed(() => props.file.displayname)
 
 const imgStyle = computed(() => {
 	if (zoomRatio.value === 1) {
diff --git a/lib/composables/useViewerProps.ts b/lib/composables/useViewerProps.ts
index 7e0da14..c488a5c 100644
--- a/lib/composables/useViewerProps.ts
+++ b/lib/composables/useViewerProps.ts
@@ -13,7 +13,7 @@ import { computed, ref, watch } from 'vue'
  * @param props The viewer props
  */
 export function useViewerProps(props: ViewerProps) {
-	const filename = computed(() => props.file.basename)
+	const filename = computed(() => props.file.displayname)
 
 	// Src is not a computed as we want to be able to change it on error.
 	// Use the encoded source so special characters in the name don't break the
diff --git a/lib/handlers.ts b/lib/handlers.ts
new file mode 100644
index 0000000..489ce76
--- /dev/null
+++ b/lib/handlers.ts
@@ -0,0 +1,287 @@
+/*!
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+import type { IFile, IFileAction, INode } from '@nextcloud/files'
+
+import FileSvg from '@mdi/svg/svg/file.svg?raw'
+import OpenInAppSvg from '@mdi/svg/svg/open-in-app.svg?raw'
+import { DefaultType, FileType, getFileActions, Permission, registerFileAction } from '@nextcloud/files'
+import { scope } from './scope.ts'
+import { logger } from './services/logger.ts'
+import { openWithHistory } from './utils/history.ts'
+import { t } from './utils/l10n.ts'
+
+/** Default click-to-open action id */
+const ACTION_VIEWER = 'viewer-open'
+/** Parent "Open with …" selector menu id */
+const ACTION_VIEWER_MENU = 'viewer-open-with'
+
+export interface IHandler {
+	/**
+	 * Unique identifier for the handler
+	 */
+	id: string
+
+	/**
+	 * The handler translated name
+	 */
+	displayName: string
+
+	/**
+	 * Optional icon for the handler
+	 */
+	iconSvgInline?: string
+
+	/**
+	 * The custom element tag name to use for this handler.
+	 */
+	tagname: string
+
+	/**
+	 * Identifier to group handlers by.
+	 * When opening a folder we'll check
+	 * against all handlers that are enabled
+	 * for the given group AND matches the
+	 * group property.
+	 */
+	group?: string
+
+	/**
+	 * Is this enabled for the given mimes ?
+	 */
+	enabled: (nodes: IFile[]) => boolean
+
+	/**
+	 * Optional function to preload data for the given node.
+	 * This will be called for the previous and next nodes on
+	 * opening a file to allow the handler to be faster when navigating.
+	 *
+	 * @param node - The node to preload data for
+	 * @return A promise that resolves when the data is preloaded
+	 */
+	preload?: (node: IFile) => Promise<void>
+
+	/**
+	 * Viewer modal theme (one of 'dark', 'light', 'default')
+	 */
+	theme?: 'dark' | 'light' | 'default'
+
+	/**
+	 * Whether this handler supports editing the current file in place.
+	 * When true the viewer shows an "Edit" action that toggles the handler's
+	 * `editing` prop (e.g. the image editor).
+	 */
+	canEdit?: boolean
+}
+
+/**
+ * Whether the viewer can open the given nodes.
+ *
+ * Answers what clicking them would do without opening anything, for a
+ * caller that has to decide whether to offer the viewer at all — a
+ * "View" button in a sidebar, say. Only files are supported, folders
+ * never match, and a set of nodes matches when one handler takes all
+ * of them.
+ *
+ * @param nodes - The node, or nodes, to test the handlers against
+ */
+export function canView(nodes: INode | INode[]): boolean {
+	return countEnabledHandlers(Array.isArray(nodes) ? nodes : [nodes], 1)
+}
+
+/**
+ * Whether at least `min` registered handlers can open the given nodes.
+ * Only files are supported, folders never match.
+ *
+ * @param nodes - The nodes to test the handlers against
+ * @param min - The minimum number of matching handlers required
+ */
+function countEnabledHandlers(nodes: INode[], min: number): boolean {
+	if (nodes.length === 0 || nodes.some((node) => node.type !== FileType.File)) {
+		return false
+	}
+
+	// Nothing to show for a file this user cannot read. Deleted files pass
+	// this: the trashbin reports them as readable, and previewing them is
+	// the point. A node that is not dav-backed always reports readable.
+	if (nodes.some((node) => (node.permissions & Permission.READ) === 0)) {
+		return false
+	}
+
+	let count = 0
+	for (const handler of getHandlers().values()) {
+		if (handler.enabled(nodes as IFile[])) {
+			count++
+		}
+		if (count >= min) {
+			return true
+		}
+	}
+	return false
+}
+
+/**
+ * Default action, triggered on file click. Opens the viewer with the first
+ * matching handler. Hidden from the actions menu to avoid cluttering it, but
+ * it is what makes any viewable file open on a single click, regardless of how
+ * many handlers are registered.
+ */
+const defaultViewerAction: IFileAction = {
+	id: ACTION_VIEWER,
+	displayName: () => t('View'),
+	iconSvgInline: () => OpenInAppSvg,
+	order: -1000,
+	default: DefaultType.DEFAULT,
+
+	enabled: ({ nodes }) => countEnabledHandlers(nodes, 1),
+	async exec({ nodes, contents, view, folder }) {
+		if (nodes[0]?.type !== FileType.File) {
+			return null
+		}
+
+		openWithHistory(contents as IFile[], nodes[0] as IFile, view, folder)
+		return null
+	},
+}
+
+/**
+ * Parent "Open with …" menu. Only shown when more than one handler can open
+ * the given nodes, so the user is offered a real choice between them.
+ */
+const openWithViewerAction: IFileAction = {
+	id: ACTION_VIEWER_MENU,
+	displayName: () => t('Open with …'),
+	iconSvgInline: () => OpenInAppSvg,
+	order: -999,
+
+	enabled: ({ nodes }) => countEnabledHandlers(nodes, 2),
+	exec() {
+		return Promise.resolve(null)
+	},
+}
+
+/**
+ * Register a new handler for the viewer.
+ * This needs to be called before the viewer is initialized to ensure the handler is available.
+ * So this should be called from an initialization script (`OCP\Util::addInitScript`).
+ *
+ * @param handler - The handler to register
+ * @throws {Error} if the handler is invalid
+ */
+export function registerHandler(handler: IHandler): void {
+	validateHandler(handler)
+
+	scope.handlers ??= new Map<string, IHandler>()
+	if (scope.handlers.has(handler.id)) {
+		logger.warn(`Handler with id ${handler.id} is already registered.`)
+		return
+	}
+
+	scope.handlers.set(handler.id, handler)
+
+	// Selector entry shown under the "Open with …" menu. Opening forces this
+	// specific handler regardless of registration order.
+	registerFileAction({
+		id: `${ACTION_VIEWER_MENU}-${handler.id}`,
+		// TRANSLATORS: handler is the translated name of the handler.
+		displayName: () => t('Open with {handler}', { handler: handler.displayName }),
+
+		iconSvgInline: () => handler.iconSvgInline ?? FileSvg,
+		parent: ACTION_VIEWER_MENU,
+		order: -999,
+
+		enabled: ({ nodes }) => {
+			if (nodes.length === 0 || nodes.some((node) => node.type !== FileType.File)) {
+				return false
+			}
+
+			return handler.enabled(nodes as IFile[])
+		},
+		async exec({ nodes, contents, view, folder }) {
+			if (nodes[0]?.type !== FileType.File) {
+				return null
+			}
+
+			openWithHistory(contents as IFile[], nodes[0] as IFile, view, folder, handler.id)
+			return null
+		},
+	})
+
+	// Register the shared actions only once.
+	const actions = getFileActions()
+	if (!actions.find((action) => action.id === ACTION_VIEWER)) {
+		registerFileAction(defaultViewerAction)
+		registerFileAction(openWithViewerAction)
+
+		logger.info('Registered viewer file actions', { id: ACTION_VIEWER, menu: ACTION_VIEWER_MENU })
+	}
+}
+
+/**
+ * Get all registered handlers.
+ */
+export function getHandlers(): Map<string, IHandler> {
+	return scope.handlers ??= new Map<string, IHandler>()
+}
+
+/**
+ * Validate the handler object.
+ *
+ * @param handler - The handler to validate
+ */
+function validateHandler(handler: IHandler): void {
+	const { id, displayName, group, enabled } = handler
+	if (typeof id !== 'string' || id.trim() === '') {
+		throw new Error('Handler id must be a non-empty string')
+	}
+
+	if (typeof displayName !== 'string' || displayName.trim() === '') {
+		throw new Error('Handler displayName must be a non-empty string')
+	}
+
+	if (typeof handler.tagname !== 'string' || handler.tagname.trim() === '') {
+		throw new Error('Handler tagname must be a non-empty string')
+	}
+
+	if (group && (typeof group !== 'string' || group.trim() === '')) {
+		throw new Error('Handler group must be a non-empty string if provided')
+	}
+
+	if (typeof enabled !== 'function') {
+		throw new Error('Handler enabled must be a function')
+	}
+
+	if (handler.preload && typeof handler.preload !== 'function') {
+		throw new Error('Handler preload must be a function if provided')
+	}
+
+	if (handler.theme && !['dark', 'light', 'default'].includes(handler.theme)) {
+		throw new Error("Handler theme must be one of 'dark', 'light', 'default' if provided")
+	}
+
+	validateCustomElementName(handler.tagname)
+}
+
+/**
+ * Validate that the given tag name is a valid custom element name.
+ *
+ * @param tagname - The custom element tag name to validate
+ */
+function validateCustomElementName(tagname: string): void {
+	if (!tagname.includes('-')) {
+		throw new Error('Handler tagname must contain a hyphen (-)')
+	}
+	if (/^[A-Z]/.test(tagname)) {
+		throw new Error('Handler tagname must not start with an uppercase letter')
+	}
+	if (/--/.test(tagname)) {
+		throw new Error('Handler tagname must not contain consecutive hyphens (--)')
+	}
+	if (tagname.startsWith('-') || tagname.endsWith('-')) {
+		throw new Error('Handler tagname must not start or end with a hyphen (-)')
+	}
+	if (!/^[a-z][a-z0-9-]*$/.test(tagname)) {
+		throw new Error('Handler tagname must only contain lowercase letters, numbers, and hyphens (-)')
+	}
+}
diff --git a/lib/helpers/handlerHelper.ts b/lib/helpers/handlerHelper.ts
index 59deb1d..7e7aae2 100644
--- a/lib/helpers/handlerHelper.ts
+++ b/lib/helpers/handlerHelper.ts
@@ -4,9 +4,9 @@
  */
 
 import type { IFile } from '@nextcloud/files'
-import type { IHandler } from '../index.ts'
+import type { IHandler } from '../handlers.ts'
 
-import { getHandlers } from '../index.ts'
+import { getHandlers } from '../handlers.ts'
 
 /**
  * Get a handler by its ID
diff --git a/lib/index.ts b/lib/index.ts
index 9d05ac8..54064ca 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -2,292 +2,11 @@
  * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
-import type { IFile, IFileAction, INode } from '@nextcloud/files'
-
-import FileSvg from '@mdi/svg/svg/file.svg?raw'
-import OpenInAppSvg from '@mdi/svg/svg/open-in-app.svg?raw'
-import { DefaultType, FileType, getFileActions, Permission, registerFileAction } from '@nextcloud/files'
 import { registerDefaultHandlers } from './defaults.ts'
-import { registerImplementation, scope } from './scope.ts'
-import { logger } from './services/logger.ts'
-import { openWithHistory } from './utils/history.ts'
-import { loadTranslations, t } from './utils/l10n.ts'
+import { registerImplementation } from './scope.ts'
+import { loadTranslations } from './utils/l10n.ts'
 import { getViewer } from './viewer.ts'
 
-/** Default click-to-open action id */
-const ACTION_VIEWER = 'viewer-open'
-/** Parent "Open with …" selector menu id */
-const ACTION_VIEWER_MENU = 'viewer-open-with'
-
-export interface IHandler {
-	/**
-	 * Unique identifier for the handler
-	 */
-	id: string
-
-	/**
-	 * The handler translated name
-	 */
-	displayName: string
-
-	/**
-	 * Optional icon for the handler
-	 */
-	iconSvgInline?: string
-
-	/**
-	 * The custom element tag name to use for this handler.
-	 */
-	tagname: string
-
-	/**
-	 * Identifier to group handlers by.
-	 * When opening a folder we'll check
-	 * against all handlers that are enabled
-	 * for the given group AND matches the
-	 * group property.
-	 */
-	group?: string
-
-	/**
-	 * Is this enabled for the given mimes ?
-	 */
-	enabled: (nodes: IFile[]) => boolean
-
-	/**
-	 * Optional function to preload data for the given node.
-	 * This will be called for the previous and next nodes on
-	 * opening a file to allow the handler to be faster when navigating.
-	 *
-	 * @param node - The node to preload data for
-	 * @return A promise that resolves when the data is preloaded
-	 */
-	preload?: (node: IFile) => Promise<void>
-
-	/**
-	 * Viewer modal theme (one of 'dark', 'light', 'default')
-	 */
-	theme?: 'dark' | 'light' | 'default'
-
-	/**
-	 * Whether this handler supports editing the current file in place.
-	 * When true the viewer shows an "Edit" action that toggles the handler's
-	 * `editing` prop (e.g. the image editor).
-	 */
-	canEdit?: boolean
-}
-
-/**
- * Whether the viewer can open the given nodes.
- *
- * Answers what clicking them would do without opening anything, for a
- * caller that has to decide whether to offer the viewer at all — a
- * "View" button in a sidebar, say. Only files are supported, folders
- * never match, and a set of nodes matches when one handler takes all
- * of them.
- *
- * @param nodes - The node, or nodes, to test the handlers against
- */
-export function canView(nodes: INode | INode[]): boolean {
-	return countEnabledHandlers(Array.isArray(nodes) ? nodes : [nodes], 1)
-}
-
-/**
- * Whether at least `min` registered handlers can open the given nodes.
- * Only files are supported, folders never match.
- *
- * @param nodes - The nodes to test the handlers against
- * @param min - The minimum number of matching handlers required
- */
-function countEnabledHandlers(nodes: INode[], min: number): boolean {
-	if (nodes.length === 0 || nodes.some((node) => node.type !== FileType.File)) {
-		return false
-	}
-
-	// Nothing to show for a file this user cannot read. Deleted files pass
-	// this: the trashbin reports them as readable, and previewing them is
-	// the point. A node that is not dav-backed always reports readable.
-	if (nodes.some((node) => (node.permissions & Permission.READ) === 0)) {
-		return false
-	}
-
-	let count = 0
-	for (const handler of getHandlers().values()) {
-		if (handler.enabled(nodes as IFile[])) {
-			count++
-		}
-		if (count >= min) {
-			return true
-		}
-	}
-	return false
-}
-
-/**
- * Default action, triggered on file click. Opens the viewer with the first
- * matching handler. Hidden from the actions menu to avoid cluttering it, but
- * it is what makes any viewable file open on a single click, regardless of how
- * many handlers are registered.
- */
-const defaultViewerAction: IFileAction = {
-	id: ACTION_VIEWER,
-	displayName: () => t('View'),
-	iconSvgInline: () => OpenInAppSvg,
-	order: -1000,
-	default: DefaultType.DEFAULT,
-
-	enabled: ({ nodes }) => countEnabledHandlers(nodes, 1),
-	async exec({ nodes, contents, view, folder }) {
-		if (nodes[0]?.type !== FileType.File) {
-			return null
-		}
-
-		openWithHistory(contents as IFile[], nodes[0] as IFile, view, folder)
-		return null
-	},
-}
-
-/**
- * Parent "Open with …" menu. Only shown when more than one handler can open
- * the given nodes, so the user is offered a real choice between them.
- */
-const openWithViewerAction: IFileAction = {
-	id: ACTION_VIEWER_MENU,
-	displayName: () => t('Open with …'),
-	iconSvgInline: () => OpenInAppSvg,
-	order: -999,
-
-	enabled: ({ nodes }) => countEnabledHandlers(nodes, 2),
-	exec() {
-		return Promise.resolve(null)
-	},
-}
-
-/**
- * Register a new handler for the viewer.
- * This needs to be called before the viewer is initialized to ensure the handler is available.
- * So this should be called from an initialization script (`OCP\Util::addInitScript`).
- *
- * @param handler - The handler to register
- * @throws {Error} if the handler is invalid
- */
-export function registerHandler(handler: IHandler): void {
-	validateHandler(handler)
-
-	scope.handlers ??= new Map<string, IHandler>()
-	if (scope.handlers.has(handler.id)) {
-		logger.warn(`Handler with id ${handler.id} is already registered.`)
-		return
-	}
-
-	scope.handlers.set(handler.id, handler)
-
-	// Selector entry shown under the "Open with …" menu. Opening forces this
-	// specific handler regardless of registration order.
-	registerFileAction({
-		id: `${ACTION_VIEWER_MENU}-${handler.id}`,
-		// TRANSLATORS: handler is the translated name of the handler.
-		displayName: () => t('Open with {handler}', { handler: handler.displayName }),
-
-		iconSvgInline: () => handler.iconSvgInline ?? FileSvg,
-		parent: ACTION_VIEWER_MENU,
-		order: -999,
-
-		enabled: ({ nodes }) => {
-			if (nodes.length === 0 || nodes.some((node) => node.type !== FileType.File)) {
-				return false
-			}
-
-			return handler.enabled(nodes as IFile[])
-		},
-		async exec({ nodes, contents, view, folder }) {
-			if (nodes[0]?.type !== FileType.File) {
-				return null
-			}
-
-			openWithHistory(contents as IFile[], nodes[0] as IFile, view, folder, handler.id)
-			return null
-		},
-	})
-
-	// Register the shared actions only once.
-	const actions = getFileActions()
-	if (!actions.find((action) => action.id === ACTION_VIEWER)) {
-		registerFileAction(defaultViewerAction)
-		registerFileAction(openWithViewerAction)
-
-		logger.info('Registered viewer file actions', { id: ACTION_VIEWER, menu: ACTION_VIEWER_MENU })
-	}
-}
-
-/**
- * Get all registered handlers.
- */
-export function getHandlers(): Map<string, IHandler> {
-	return scope.handlers ??= new Map<string, IHandler>()
-}
-
-/**
- * Validate the handler object.
- *
- * @param handler - The handler to validate
- */
-function validateHandler(handler: IHandler): void {
-	const { id, displayName, group, enabled } = handler
-	if (typeof id !== 'string' || id.trim() === '') {
-		throw new Error('Handler id must be a non-empty string')
-	}
-
-	if (typeof displayName !== 'string' || displayName.trim() === '') {
-		throw new Error('Handler displayName must be a non-empty string')
-	}
-
-	if (typeof handler.tagname !== 'string' || handler.tagname.trim() === '') {
-		throw new Error('Handler tagname must be a non-empty string')
-	}
-
-	if (group && (typeof group !== 'string' || group.trim() === '')) {
-		throw new Error('Handler group must be a non-empty string if provided')
-	}
-
-	if (typeof enabled !== 'function') {
-		throw new Error('Handler enabled must be a function')
-	}
-
-	if (handler.preload && typeof handler.preload !== 'function') {
-		throw new Error('Handler preload must be a function if provided')
-	}
-
-	if (handler.theme && !['dark', 'light', 'default'].includes(handler.theme)) {
-		throw new Error("Handler theme must be one of 'dark', 'light', 'default' if provided")
-	}
-
-	validateCustomElementName(handler.tagname)
-}
-
-/**
- * Validate that the given tag name is a valid custom element name.
- *
- * @param tagname - The custom element tag name to validate
- */
-function validateCustomElementName(tagname: string): void {
-	if (!tagname.includes('-')) {
-		throw new Error('Handler tagname must contain a hyphen (-)')
-	}
-	if (/^[A-Z]/.test(tagname)) {
-		throw new Error('Handler tagname must not start with an uppercase letter')
-	}
-	if (/--/.test(tagname)) {
-		throw new Error('Handler tagname must not contain consecutive hyphens (--)')
-	}
-	if (tagname.startsWith('-') || tagname.endsWith('-')) {
-		throw new Error('Handler tagname must not start or end with a hyphen (-)')
-	}
-	if (!/^[a-z][a-z0-9-]*$/.test(tagname)) {
-		throw new Error('Handler tagname must only contain lowercase letters, numbers, and hyphens (-)')
-	}
-}
-
 // Offer this copy as the page's viewer. Registering costs nothing: the
 // implementation chunk is only fetched by whichever copy wins, and only
 // once something actually opens a file.
@@ -314,6 +33,8 @@ getViewer()
 // renders, and a handler registered after that is a file that does not open.
 registerDefaultHandlers()
 
+export { canView, getHandlers, registerHandler } from './handlers.ts'
+export type { IHandler } from './handlers.ts'
 export { getViewer, Viewer } from './viewer.ts'
 export type { ViewerAPI, ViewerEmits, ViewerOptions, ViewerProps } from './viewer.ts'
 export { registerDefaultHandlers } from './defaults.ts'
diff --git a/lib/models/audios.ts b/lib/models/audios.ts
index 0718099..c6d5bfc 100644
--- a/lib/models/audios.ts
+++ b/lib/models/audios.ts
@@ -5,7 +5,7 @@
 
 import AudioOutlineSvg from '@mdi/svg/svg/music-note-outline.svg?raw'
 import { defineCustomElement } from 'vue'
-import { registerHandler } from '../index.ts'
+import { registerHandler } from '../handlers.ts'
 import { logger } from '../services/logger.ts'
 import { t } from '../utils/l10n.ts'
 
diff --git a/lib/models/images.ts b/lib/models/images.ts
index 57be0b5..04aefcf 100644
--- a/lib/models/images.ts
+++ b/lib/models/images.ts
@@ -5,7 +5,7 @@
 
 import { getCapabilities } from '@nextcloud/capabilities'
 import { defineCustomElement } from 'vue'
-import { registerHandler } from '../index.ts'
+import { registerHandler } from '../handlers.ts'
 import { logger } from '../services/logger.ts'
 import { t } from '../utils/l10n.ts'
 
diff --git a/lib/models/videos.ts b/lib/models/videos.ts
index acf1b7f..8e28453 100644
--- a/lib/models/videos.ts
+++ b/lib/models/videos.ts
@@ -5,7 +5,7 @@
 
 import MovieOutlineSvg from '@mdi/svg/svg/movie-outline.svg?raw'
 import { defineCustomElement } from 'vue'
-import { registerHandler } from '../index.ts'
+import { registerHandler } from '../handlers.ts'
 import { logger } from '../services/logger.ts'
 import { t } from '../utils/l10n.ts'
 
diff --git a/lib/scope.ts b/lib/scope.ts
index 459f0c1..709d245 100644
--- a/lib/scope.ts
+++ b/lib/scope.ts
@@ -2,7 +2,7 @@
  * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
-import type { IHandler } from './index.ts'
+import type { IHandler } from './handlers.ts'
 import type { Viewer } from './viewer.ts'
 
 import { logger } from './services/logger.ts'
diff --git a/lib/viewer.ts b/lib/viewer.ts
index 8dfb512..fd4cae6 100644
--- a/lib/viewer.ts
+++ b/lib/viewer.ts
@@ -136,6 +136,13 @@ export type ViewerOptions = {
 	 */
 	canLoop?: boolean
 
+	/**
+	 * Whether to offer the Files sidebar for the open file. Defaults to true.
+	 * Turn it off for a file the sidebar cannot resolve, such as an old
+	 * version of a file, which is served from its own dav endpoint.
+	 */
+	enableSidebar?: boolean
+
 	/**
 	 * The files view the viewer was opened from. Forwarded to the file actions
 	 * rendered inside the viewer (download, delete, details, …) so they can run
@@ -155,6 +162,7 @@ const defaultViewerOptions: ViewerOptions = {
 	onNext: () => {},
 	onClose: () => {},
 	canLoop: true,
+	enableSidebar: true,
 }
 
 export interface ViewerAPI {
diff --git a/lib/views/Viewer.vue b/lib/views/Viewer.vue
index 7fc1e27..3e5d260 100644
--- a/lib/views/Viewer.vue
+++ b/lib/views/Viewer.vue
@@ -55,7 +55,7 @@
 
 			<!-- Open sidebar for the current file -->
 			<NcActionButton
-				v-if="!isSidebarShown && !!currentFile"
+				v-if="!isSidebarShown && !!currentFile && canOpenSidebar"
 				closeAfterClick
 				@click="showSidebar">
 				<template #icon>
@@ -202,7 +202,7 @@
 <script setup lang="ts">
 import type { IFile, IFolder, INode, IView } from '@nextcloud/files'
 import type { IFileAction } from '@nextcloud/files'
-import type { IHandler } from '../index.ts'
+import type { IHandler } from '../handlers.ts'
 import type { ViewerAPI, ViewerOptions } from '../viewer.ts'
 
 import { showError } from '@nextcloud/dialogs'
@@ -224,8 +224,8 @@ import FullscreenIcon from 'vue-material-design-icons/Fullscreen.vue'
 import FullscreenExitIcon from 'vue-material-design-icons/FullscreenExit.vue'
 import PencilIcon from 'vue-material-design-icons/Pencil.vue'
 import { useViewerActions } from '../composables/useViewerActions.ts'
+import { getHandlers } from '../handlers.ts'
 import { getHandlerForFile } from '../helpers/handlerHelper.ts'
-import { getHandlers } from '../index.ts'
 import { fetchFolderContent } from '../services/dav.ts'
 import { logger } from '../services/logger.ts'
 import { canDownload } from '../utils/canDownload.ts'
@@ -283,12 +283,17 @@ const canEdit = computed(() => currentHandler.value?.canEdit === true
 	&& ((currentFile.value?.permissions ?? Permission.NONE) & Permission.UPDATE) !== 0)
 const currentOptions = ref<ViewerOptions>({
 	canLoop: true,
+	enableSidebar: true,
 	onClose: () => {},
 	onNext: () => {},
 	onPrev: () => {},
 	loadMore: () => Promise.resolve([]),
 })
 
+// The sidebar resolves a file by its dav source, so it can only be offered
+// for a file the Files app can find there.
+const canOpenSidebar = computed(() => currentOptions.value.enableSidebar !== false)
+
 // Comparison context (compare API)
 const comparisonFile = ref<IFile>()
 const comparisonHandler = ref<IHandler>()
@@ -490,11 +495,11 @@ watch(editing, (value) => {
 const modalName = computed(() => {
 	if (isComparing.value) {
 		return t('Comparing {file1} and {file2}', {
-			file1: currentFile.value?.basename ?? '',
-			file2: comparisonFile.value?.basename ?? '',
+			file1: currentFile.value?.displayname ?? '',
+			file2: comparisonFile.value?.displayname ?? '',
 		})
 	}
-	return currentFile.value?.basename || ''
+	return currentFile.value?.displayname || ''
 })
 
 const hasNext = computed(() => {
@@ -1074,7 +1079,7 @@ watch(currentFile, async (newFile, oldFile) => {
 	openedSubmenu.value = null
 	// Here rather than on open, so paging to the next file retitles the page too
 	if (newFile) {
-		setViewerTitle(newFile.basename)
+		setViewerTitle(newFile.displayname)
 	}
 	if (newFile && !oldFile) {
 		await nextTick()