Xbox Video app has received a new update for Windows 8.1 devices. The new version is 2.6.0432.0. The update brings MKV support , along with several fixes.
Xbox Video Changelog:
Support for MKV videos (the extension used for the Matroska Media Container). The update supports playback of most MKV files as long as they contain streams that use codecs supported by the app. For more information on MKV support on Windows, please check out the related article about support for MKV on Xbox One. The implementation is the same on both platforms except that V_MPEG2 is only supported on the console.
A fix that makes our Spotlight section more robust
A fix for an issue that caused the Rent button to appear when viewing TV series and shows
A fix for an issue that was causing file metadata to appear in the video title in your collection
Fixes for two issues in deeplinking to content within Xbox Video from other apps (the movie details page and playing trailers, specifically)