The .mkv extension (Matroska Video) indicates this is a high-quality video file that can hold multiple tracks (audio, subtitles, etc.).
If you have this file and want to know what it is without playing it, you can try the following: AgADmwADcXuhVg.mkv
Files named this way are usually generated when someone uploads a video to a cloud service or messaging app. How to identify the content: AgADmwADcXuhVg.mkv