bug#62815: Gnus: MIME types of attachments contain regexp quotation

[Torsten Bronger]

Hallöchen!

Since commit 3faa508eba84a1983732099cbd3cc1eaad404158, I observe
incorrect MIME types in my outgoing mails, e.g.

…
--=-=-=
Content-Type: application/vnd\.oasis\.opendocument\.spreadsheet
Content-Disposition: attachment;
 filename=2022-12-15_Nachbestellung_Studio.ods
Content-Transfer-Encoding: base64
…


Reason is that when attaching a file to a mail in Gnus, the MIME
type is suggested by Gnus, and this suggestion contains the
incorrect backslashes.

Regards,
Torsten.

-- 
Torsten Bronger