RFC/PATCH emacs attachment handling

[Mark Walters]

(Re-sent since accidentally didn't include list in reply)

> This is great! In particular many people send PDF attachments to me with
> mime-type attachment/octet-stream, and notmuch could not view them and
> only offered to save them... I have wanted this functionality for long!

In fact in most cases this patch "will just work": that is <enter> on
the button will open the file as if it were application/pdf. Notmuch has some
logic to guess the mime type of application/octet-stream from the file
extension and this patch uses that.

The view all attachments command (which is unchanged by this patch)
uses emacs mime handling directly so does not use notmuch's guess.

Best wishes

Mark