Windows – Thunderbird uses wrong attachment mime type on Windows Vista

Thunderbird running on Windows Vista has usually sent Microsoft Word documents as mime type application/msword, which as I understand is correct. However, now it has suddenly decided to send Word documents as mime type text/richtext, which creates a big problem as a recipient client may interpret the message as displayable inline – which in fact just creates garbage on the screen as it cannot handle direct display of Word documents. At least Thunderbird itself as a recipient does this, but I'd imagine other clients might as well.

As far as I can understand, Thunderbird picks up the mime type of the attachment from the operating system, instead of determining it by itself. This would suggest that something might be wrong with how Vista assigns mime types. However, I coudn't find a way to adjust on Vista how mime types are determined – only file type setting seems to be about which program opens them. However, this seems to be in order, since in registry HKEY_CLASSES_ROOT\.doc does contain the correct mime type – and in fact, the string text/richtext is nowhere to be found in registry.

It's also worth mentioning that other file types do still get their correct mime type.

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

Or: How do I determine what mime types are assigned to different file types on Windows Vista?

Or: How do I troubleshoot this issue further? (For example by determining if the problem is with Vista providing the wrong mime type for Thunderbird, or with Thunderbird not accepting what vista provides?)

Here's a sample of the failed attachment header:

Content-Type: text/richtext; name="Foo Bar.doc"    
Content-Description: Foo Bar.doc    
Content-Disposition: attachment;    
 filename="Foo Bar.doc"; size=85056;    
 creation-date="Thu, 17 Sep 2009 11:14:20 GMT";    
 modification-date="Thu, 17 Sep 2009 11:14:20 GMT"    
Content-Transfer-Encoding: base64

The problem now has a solution. The immediate cause of the problem turned out to be mimeTypes.rdf. The problem was caused by (and can be duplicated by adding) the following section in the file:

<RDF:Seq RDF:about="urn:mimetypes:root">
  <RDF:li RDF:resource="urn:mimetype:text/richtext"/>
</RDF:Seq>
<RDF:Description RDF:about="urn:mimetype:text/richtext"
                 NC:value="text/richtext"
                 NC:editable="true"
                 NC:fileExtensions="doc"
                 NC:description="Microsoft Office Word 97 - 2003">
  <NC:handlerProp RDF:resource="urn:mimetype:handler:text/richtext"/>
</RDF:Description>