emacs: Use mailcap.el to guess the type of application/octet-stream parts

Use the mailcap functionality to guess a MIME type for attachments of
type application/octet-stream and, presuming successful, feed the
attachment back into the display code with the determine type.

This is mostly useless at the moment, as the JSON output from notmuch
does not include the content of application/octet-stream parts, so
they cannot be displayed even if the guess is a good one.