missing piece for evince?

["Steven M. Bellovin" <smb%cs.columbia.edu@localhost>]

On one of my machines, evince can open (most) pdf files without any
trouble.  On two others, it says "Unable to open docuemnt: Unhandled
MIME type "text/plain".  It does the same if I select the document via
the File menu.  However -- and this is where it gets weird -- if I
select File|Open, the offending document is pre-selected and opens
properly...

I assume I'm missing some file mapping extensions to filetypes.  I have
no idea which, and ktrace wasn't particularly helpful.  I have no idea
why, after the initial failure, it was able to ascertain the type
correctly.  I also have no idea why it works properly on one machine
but not on others.  (Note that none of the three run Gnome.)  The
working machine is amd64-current; the other two are i386-current.

                --Steve Bellovin, http://www.cs.columbia.edu/~smb