Adds file property to WebAudioMedia for access to downloaded audio file data.

[probityrules]

It would be useful to make the raw audio data available from WebAudioMedia so things like meta ID3 tags can be accessed without requiring additional network calls. Here is my take, although I'm not sure if this is the best route for other use-cases. If not, it would still be great to have some way to access the response ArrayBuffer apart from this class's immediate conversion to an AudioBuffer.

If there's a standard way to extend `WebAudioMedia" and have Sound use the extended class instead, that could work as well: I just wasn't sure about how to approach that.