@WOX, that's not an easy question to answer. The file extension doesn't necessarily mean anything - it's what's in the file that matters. There's no easy way to get that information other than using another media player on a computer that can give the video's properties or a utility like MediaInfo. Roku devices support a limited number of codecs. You can see the list here:
How to use Roku Media Player to play your videos, music and photos | Official Roku Support
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