Various minor changes related to file mime type detection.

- Improve or just remove some file magic signatures ported from libmagic
  that were too general and matched incorrectly too often.

- Fix MHR script's use of fa_file$mime_type before checking if it's
  initialized.  It may be uninitialized if no signatures match.

- The "fa_file" record now contains a "mime_types" field that contains
  all magic signatures that matched the file content (where the
  "mime_type" field is just a shortcut for the strongest match).