[ticket/11912] Add mimetype guesser for proper mimetype guessing

This will first of prevent us hitting the LogicException that we would hit by just using the Symfony MimeTypeGuesser as described in the initial ticket:
http://tracker.phpbb.com/browse/PHPBB3-11912

Additionally, we'll now have a last fallback if no mimetype could be guessed with any of the available MimeTypeGuessers or even the function mime_content_type(): The content_guesser will fall back to mapping the file extensions of the supplied file to a mimetype.