Added hintForMimeType: method

Several years later, I really should get around to submitting this as a pull request!

I've added a hintForMimeType: method which returns an appropriate AudioFileTypeID hint for a valid audio file format MIME type, as suggested in the current codebase comments. Also modified handleReadFromStream:eventType: to utilise this new method.

I retained the old hintForFileExtension: method in the code base as it is perfectly functional and, in some cases, may be preferred to using the MIME type detection.