Bugzilla Bug 170

When doing filtering it would be nice to have a couple of other fields
available. One is a Mimetype field and the other is a field to check whether
the message has been marked as spam.

I would use the mimetype field to filter out messages that contain picture
attachments that have no extension. These images are still displayed because
they have a valid mimetype, but I am unable to filter them out since I can't
check the file extension.

The Marked as Spam field would be useful to me because I find some messages are
diverted from the spam folder by filters. If this field was available, I could
simply add a 'AND NOT MarkedAsSpam' clause to my filters.