.mkv — Matroska Video
Video video/x-matroska
What a .mkv file is
An open container that can hold essentially any codec, plus multiple audio tracks, subtitles and chapters in one file.
How to open it
VLC, MPV, Plex. Browsers and phones generally will not play it directly.
Worth knowing
The format of choice for archiving, because it puts no constraints on what it carries.