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).

scripts/base/frameworks/files/magic/general.sig

@@ -6,6 +6,12 @@ signature file-plaintext {
 }
 
 signature file-binary {
-    file-magic /(.*)([^[:print:][:space:]]+)/
+    # Exclude bytes that can be ASCII or some ISO-8859 characters.
+    file-magic /(.*)([^[:print:][:space:]\xa0-\xff]+)/
     file-mime "binary", -10
 }
+
+signature file-tar {
+    file-magic /([[:print:]\x00]){100}(([[:digit:]\x00\x20]){8}){3}/
+    file-mime "application/x-tar", 150
+}