Better mime types for attachments. Use some command; e.g. `file --mime'.

Better mime types for attachments. Use some command; e.g. `file --mime'.
Up to now attachments had either been text/plain or application/octet-stream,
if one hadn't added mhshow-suffix- profile entries for any possible type.
Now we call mimetypequeryproc (default: `file -b --mime' for GNU file),
this can be changed with the profile entry `Mime-Type-Query'. This command
will be executed with the attachment filename as last argument. If your
file(1) command does not print MIME types, you can still grep your profile
for matching mhshow-suffix- entries. However, I suggest to store this data
somewhere else. IMO the profile is not the right location for it.