By sniffing specifically for MP4 and WebM, we were precluding
PlaybackManager from playing any other formats. Instead, use
MatroskaDemuxer if the media has a `matroska` or `webm` EBML doctype,
and fall back to FFmpeg for all others.
We'll need to limit the containers that FFmpeg is able to open at some
point, but for now, this allows us to play the formats we could before.
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