#application/vnd.vnd.sun.xml.impress; name="foo.sxi" \
/home/bart/foo.sxi
Now you do a little dance with MH "whatnow" and you're set. But what an error-prone typing nightmare! OK, here's my 15-minute solution, after years of whining about the problem. Comes pretty close IMHO!
First, cobble together some emacs lisp code (attached) that will let you hit \C-c\C-a in emacs MH message editing mode, prompt you for the file to be attached, invoke a shell script with the filename to get it expanded to the right mystic nonsense, and then insert that nonsense into the buffer.
Then, write the shell script (attached) that does the expansion, by grubbing through /etc/mime.types.
I'm not sure whether I'm cool, or the world's biggest idiotdweeb, but I'm sure enjoying my new toy.