Don't always try and autodetect the content type
Due to legacy reasons, we try and detect the content type of the media file we are trying to load. This was originally because we always used to transcode the media file. But this isn't something we should always try and do.
We should only try and detect the content type when we are trying to transcode the media file, otherwise we can just send the file or URL to the chromecast and it can detect the content type and play the media file if possible.
Related issues: https://github.com/vishen/go-chromecast/issues/79 and https://github.com/vishen/go-chromecast/issues/96. Neither of these would be an issue if we didn't try and guess the content type and return an error if we couldn't.