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AVI files should be guessed too.
http://www.jmcgowan.com/avitech.html#Format http://www.the-labs.com/Video/odmlff2-avidef.pdf 52 49 46 46 00 00 00 00 41 56 49 20 FF FF FF FF 00 00 00 00 FF FF FF FF None. video/avi The string "RIFF" followed by four bytes followed by the string "AVI ". https://github.com/whatwg/mimesniff/commit/10a2a3afa92b27be54fc92c6d37e67bb3e9b64be