Since this guesser is registered last, it will be the first one run. This guesser will always return the contents of mime_content_type or application/octetstream, so no other guessers would ever be run. null must be returned here if mime_content_type cannot be run.

If we need to ensure application/octet-stream isn't being saved as the mimetype we must check the output of guesser->guess() and fix the output.