In reading the FAQ at http://www.python.org/cgi-bin/faqw-mm.py?req=show&file=faq04.046.htp I came to the impression that the mime-type and extension must match. If they don't the extension is changed to .bin.
I checked /etc/mime.types on my etch system and there is no text/x-patch. 'diff' is listed as a file type under text/plain.
As an experiment I am attaching an ASCII text file to this e-mail with a .txt and a .diff extention, each file will get a mime type of text/plain and text/x-patch.
Let's see what mailman does...
Ron