Fix broken string literal that disabled the quoted-printable attachment branch

[chuenchen309]

In core.py's parse(), the branch that keeps quoted-printable application/*
attachments as binary has a broken string literal:

elif transfer_encoding == "base64" or (
    transfer_encoding
    == "quoted-\
                   printable"
    and "application" in mail_content_type
):

The backslash is a line continuation, so the continuation line's indentation
becomes part of the string — the comparison target is
"quoted- printable", which never equals
"quoted-printable". That branch is dead.

As a result a quoted-printable application/* attachment falls through to the
text branch and is decoded as UTF-8 with errors="ignore", silently dropping
every non-UTF8 byte and setting binary=False. A 16-byte binary attachment
comes back as 9 bytes; the original file is unrecoverable.

Joining the literal to == "quoted-printable" restores the intended
binary-preserving path (the base64 operand next to it is already a clean
single-line literal). Added a test with a quoted-printable
application/octet-stream attachment that now round-trips losslessly; the
existing text/plain quoted-printable test is unaffected (it stays
binary=False), which is why the bug was never caught.

---

This PR was authored by an AI coding agent (Claude Code) running on this account:
the AI found the bug, ran the repro, wrote the test, and wrote this description.
The human account holder reviews every change and is accountable for it. The
verification is real and re-runnable from the diff. If this isn't the kind of
contribution you want, say so and I'll close it.