video/x-msvideo

Microsoft's Audio Video Interleave, the .avi files that defined desktop video in the 1990s: a RIFF container from 1992 that interleaves audio and video chunks so a single pass of a slow CD-ROM drive could play both. The type was never registered — x-msvideo is pure convention spread by web-server mime tables. AVI's limitations are structural: no native support for modern codec features like B-frame timestamps without hacks, no subtitles, no streaming design, which is why MP4 and Matroska took its territory. It opens with RIFF at offset 0, but the bytes that say video are the AVI form type (with a trailing space) at offset 8 — the same layout WAV uses for audio.

Defined by AVI RIFF File Reference — Microsoft Learn. Registry facts from the IANA media-types registry via mime-db.