Linux – Fixing Inconsistent Default MIME Type Between System and Mozilla Firefox/Thunderbird

Interesting issue. Upfront it's worth noting that text/richtext is an e-mail related mime type obsoleted by text/enriched and entirely unrelated to application/rtf, see Wikipedia about Enriched text:

Enriched text is a formatted text format for e-mail, defined by the IETF in RFC 1896 and associated with the text/enriched MIME type.
[...]
A predecessor of this text/enriched MIME type was called text/richtext in RFC 1341 and RFC 1521. Neither should be confused with Rich Text Format (MIME type text/rtf or application/rtf) which is an entirely different specification, devised by Microsoft.

The only clue I could find is targeting Horde/IMP via Firefox, but seems to address the same problem, see Word attachment file problem:

You might have received some document which is in rich text format. You might have opened it using Microsoft word and this has resulted in adding a mime-type called 'text/richtext' with its supporting application as MS-word in your browser setting. Subsequently, when you attach any word document, the browser is identifying any attached word document as 'text/richtext' type instead of 'application/msword'.

Due to Firefox and Thunderbird sharing much code this might well apply to Thunderbird too.

How do I force Thunderbird to use the correct application/msword mime type for Microsoft Word document attachments (when sending email)?

Why this is happening has apparently not been finally resolved, however, the temporary solution offered by the poster should apply too, as Thunderbird is using mimeTypes.rdf also. However, before applying the drastic measure to simply delete this file from your Thunderbird user profile and restart Thunderbird (which will likely restore the default file), I'd try to investigate this further to avoid loosing deliberate changes to Download Actions:

• First you should have a look into your Thunderbird settings:
  Under Tools->Options->Attachments->Download Actions you'll find the user configured mime type related actions per file extension as stored in mimeTypes.rdf (i.e. just those different from default settings I guess). In principle the changed setting regarding Word documents should manifest itself there (though given the circumstances it might as well be some implied specialty or even a bug).
• In case this doesn't yield a result you could still peak into mimeTypes.rdf and try to figure out which particular setting is wrong, if any.

Whether avoiding the simple deletion of mimetypes.rdf like so is worth the trouble depends on your situation of course.

You could find some use in these extensions:

• MIME Edit (modified version)
• Open attachment by extension

I use them on Windows, mind you, but they're supposed to be cross-platform. The first one is just an improved way of handling attachments by MIME type ("improved" over TB's default UI), while the second one allows you to ignore the MIME type altogether and set the opening application just based on the file extension - it's useful for those e-mails where the attachment has the wrong MIME type (sender's fault).

Note that the first extension isn't available on AMO anymore; the link points to a user-modified version, as suggested here.