Matroska Video Stream is a popular container file for media, like movies and TV shows and is used widely on the internet. The feature was mentioned in our comments by brdloch, and we were able to confirm this observation this morning on a Lumia 930 running Windows Phone 8.1 build 15116. Best VPN providers 2020: Learn about ExpressVPN, NordVPN & more To test for the feature, we copied over a 5 MB MKV video file sample at 1280 x 536 resolution to two phones: Lumia 930 with Update 2 and a Lumia 929 (Icon) with Update 1 and Lumia Denim. Windows asks if you want to convert the file, to which we answered 'no' on both occasions.