Loosen suffix enforcement

At the minute a local file must use the exact format suffix (`flv` or `mp4`). This commit loosens that constraint, allowing a prefix to be used to specify the format and the filename to not use the standard format suffix (for instance, `mp4:test.mov`. The existing behaviour is preserved, it tries `test.mov.mp4` and then falls back on trying `test.mov` (the `.mov` extension I can see as a valid use case, in my particular case I do not have a file extension at all)

The only issue I can see (albeit with my limited experience of the pecularities of RTMP clients) is that it would be possible for a malicious user to force parsing an FLV as MP4 or vice versa