Bug#1009181: File suffix for message mbox links should be .eml

On Fri, 08 Apr 2022, Max Nikulin wrote:
> Notice that the default file name to save the message has ".mbox"
> suffix while freedesktop mime database entry assumes ".eml" for the
> "message/rfc822" mime type:
> https://sources.debian.org/src/shared-mime-info/1.10-1/freedesktop.org.xml.in/#L5466
> As a result if "message/rfc822" mime type is associated with
> thunderbird.desktop file then the application starts composition of a
> new message with the .mbox file as attachment instead of displaying
> the downloaded message.


> I have not managed to convince thunderbird developers that there
> should be a way to tell the application that some file should be
> opened namely as a message no matter what is the name and the
> extension of the file. They believe that current heuristics is correct
> and ".mbox" suffix means collection of messages in "application/mbox"
> mail box. Thunderbird does not support opening a mbox file outside of
> its mail directories.

The only real difference between a single message in an mbox and a
complete mbox is From escaping. Debbugs is probably technically
incorrect, because we do From escaping even if we're returning just a
single message, so we're actually returning an application/mbox instead
of a message/rfc822.

Frankly, using the extension to determine mime time is bad practice, but
it's a common bad practice.

I've no objection to changing the default extension; I personally wasn't
aware of the eml extension when I wrote that part of the code.

We should also not do From escaping when we're just returning a single
message.