There's mimetypes that easily fool mimetype detection (eg. OGG videos with
.ogg extension instead of .ogv will be detected as audio/ogg), the
GstDiscoverer will however find out correctly whether there's audio and/or
video information, so we should rely on it as a last resort, rather than
(weaker) mimetype sniffing.

This prevents .ogg suffixed videos from being played by gnome-music (oddly,
with success, in a separate window).