Change FLAC MIME type

Change streaming FLAC MIME type from application/octet to audio/flac to
support Frontier Silicon systems (such as the Axis X3) that require
this. (Others don't care.)

Only affects UPNP streaming.

(Might be worth updating the Neptune / Platinum code as well. It is currently running 0.6.9.1 but the latest is 1.0.4.11.)