Don't allow subject to be changed when forwarding file attachments

Since the filename is in the subject, we can't allow the user to change the subject (duh). Also, don't bother with the "Fwd:" prefix thing as that could potentially cause problems in other places where the filename (only) is assumed to be in the subject.

Also, make sure to create the destination directory when moving file attachments. These changes fix issue #230 reported by Keyop.