video/x-flv

The container that carried early YouTube: FLV, introduced with Flash Player 6 in 2002, made in-browser video practical years before the video element existed, streaming H.263, VP6, and eventually H.264 through the Flash plugin. Its fate tracked its host's — when Flash died in 2020, FLV lost its player, and the format survives mainly in one unlikely niche: RTMP, the companion streaming protocol, still moves live broadcasts to ingest servers with FLV-framed payloads. The type was never registered. Files open with the three ASCII bytes FLV followed by a version byte, one of the tidiest signatures in the registry.

.flv is the FLV container; .f4v is later Flash video in an MP4 container — different bytes, same ecosystem.

Defined by Flash Video — Wikipedia. Registry facts from the IANA media-types registry via mime-db.