[ticket/13031] Only use mimetype guesser guess if it helps us

If we already have a mimetype and the guesser's guess is the default fallback,
we should keep the already existing mimetype the browser supplied. Otherwise,
platforms that might not support mimetype guessers will cause us to always
have the mimetype set to application/octet-stream on images. This will prevent
users from uploading images.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-13031