application/mp4

The MP4 nobody watches: RFC 4337 reserves this for MP4 files with neither an audio nor a visual presentation — pure MPEG-4 systems streams, scene descriptions, that sort of exotica. In the wild it mostly appears as a misconfiguration, a server hedging on a video file it should have called video/mp4; browsers may download it instead of playing it, since application/* invites a save dialog. Structurally it is the same ISO container, ftyp box at offset 4, brand bytes varying after it.

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