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

[Eli Zaretskii]

> From: Torsten Bronger <bronger@physik.rwth-aachen.de>
> Date: Thu, 13 Apr 2023 13:46:38 +0200
> 
> 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.

Lars, could you please look into this?