video/x-matroska

The container that holds anything — MKV files routinely carry HEVC video, multiple audio languages, and full subtitle tracks, which is why the piracy scene and the archival world both settled on it. The x- prefix marks this as the unregistered de facto name; Matroska itself is now formally specified in RFC 9559, but browsers still do not play it natively, so it stays a download-and-open format. The EBML signature at offset 0 is the same four bytes WebM uses — WebM is a Matroska subset — and only the DocType element inside separates them.

.mkv is video, .mka audio, .mks subtitles-only, .mk3d stereoscopic — one container, four suffixes.

Defined by RFC 9559. Registry facts from the IANA media-types registry via mime-db.