video/x-ms-wmv

Windows Media Video: Microsoft's codec family from the DRM-optimistic 2000s, riding inside ASF, the proprietary Advanced Systems Format container that also carries WMA audio. The pairing powered Windows Media Player's era and a generation of protected downloads, then faded as H.264-in-MP4 became the universal answer; the x- type was never registered, though Microsoft did register the container itself as application/vnd.ms-asf. Identification is unmistakable: every ASF file opens with the same 16-byte header-object GUID, one of the few formats whose magic number is a literal Windows GUID rather than ASCII.

.wmv, .wma, and .asf are all the same ASF container — the extension hints at the payload, the bytes do not differ.

Defined by Overview of the ASF Format — Microsoft Learn. Registry facts from the IANA media-types registry via mime-db.