The .mkv extension (Matroska Video) indicates this is a high-quality video file that can hold multiple tracks (audio, subtitles, etc.).
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
If you have this file and want to know what it is without playing it, you can try the following: AgADmwADcXuhVg.mkv
Use a tool like MediaInfo to see the internal title, encoding date, or tags that might be hidden inside the file. AgADmwADcXuhVg.mkv