bug#35507: Gnus mojibakifies UTF-8 text/x-patch attachments from Thunderbird

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Thu, 02 May 2019 08:01:38 -0400
> Cc: 35507@debbugs.gnu.org
> 
> I understood from Paul's followup in https://debbugs.gnu.org/35507#32
> that the report is mainly about the case where there is a Content-Type
> header with a charset field within the body of the attachment.

Yes, that's my understanding as well.  So I guess Gnus should try
gleaning the charset from there.  The 'undecided' stuff is for when it
fails, I think.