HOW TO: Enable/fix playback of various media formats in Opus has suggested codecs if you need them.
Once the files play OK in Microsoft's video player (Film & TV or similar; they seem to rename it every few months), the thumbnails should work in both Opus and Explorer. Their video player will sometimes tell you which coded is needed, although I'd usually recommend installing all of the "Codecs provided by Microsoft" from the guide I linked to, to save time.