Also backported using the longer file switches (--mime and --breif) because the short ones (-ib) aren't the same in all versions of file.

* Use the 'mimedetect_file_binary' variable instead of 'filefield_file_binary'.
* Use a $t() instead of t() in the requirements function.
* Merge the last two mime type checks.
* Move the declaration of the $mime variable further down for better
  readability, and remove an unnecessary check on !$mime.
* Add the mime type for png, and fix ico and rgb (was 'rbg' before).
* Tune user visible strings for sounding "official" and consistent casing.
* Remove trailing spaces.