Fix bug in miemetype name comparator

The code was removing the first matching prefix (x- or x-ms-), which
caused 'x-ms-bmp' to end up as 'ms-bmp'. It should be 'bmp'. Fixed by
reverting the order of tokens in the regexp.

program/steps/mail/get.inc

@@ -311,7 +311,7 @@ exit;
  */
 function rcmail_mimetype_compare($type1, $type2)
 {
-    $regexp = '~/(x-|x-ms-)~';
+    $regexp = '~/(x-ms-|x-)~';
     $type1  = preg_replace($regexp, '/', $type1);
     $type2  = preg_replace($regexp, '/', $type2);