The extension indicates a Matroska Video file. It is not a video codec itself but a "multimedia container" that can bundle an unlimited number of video, audio, picture, and subtitle tracks into a single file.
Its name is derived from the Russian Matryoshka (nesting) dolls, symbolizing how multiple streams are "nested" inside the container. AgAD0gADBUChVw.mkv
You may encounter specific challenges when dealing with files named in this format from Telegram: The extension indicates a Matroska Video file