video/mp4

The default video type of the web — the ISO base media file format carrying whatever codecs the producer chose, and that is the catch: video/mp4 promises a container, not a codec. An MP4 can hold H.264, HEVC, AV1, or VP9, and only the codecs parameter (codecs="avc1.42E01E, mp4a.40.2") tells a player whether it can decode before it downloads. The signature is the ftyp box at offset 4, preceded by a size field; the brand bytes after it (isom, mp42, avc1) vary and are not worth matching on.

.mp4 everywhere; .m4v is Apple's flavor, historically implying DRM or iTunes metadata.

Defined by RFC 4337 · IANA registration. Registry facts from the IANA media-types registry via mime-db.