Here's a little something for Haiku: /bin/setmime -set video/x-matroska -short "Matroska Video" -sniffRule '0.50 (0x1A45DFA3934282886D6174726F736B61428781014285810118538067)' - extension mkv /bin/setmime -set audio/x-matroska -short "Matroska Audio" -sniffRule '0.50 (0x1A45DFA3934282886D6174726F736B61428781014285810118538067)' - extension mka http://www.matroska.org/technical/specs/notes.html: " There is no IETF endorsed MIME type for Matroska files. But you can use the ones we have defined on your system : .mka : Matroska audio audio/x-matroska .mkv : Matroska video video/x-matroska " There's also some "application/x-matroska" in the wild, but that is not endorsed by the Matroska project. The sniffing rule I got from http://filext.com/detaillist.php?extdetail=MKV I don't know if the audio files need a separate one as I haven't been able to find any .mka files. Q1: Filename extensions are case-insensitive to Haiku, right? (It would be bad to not catch all permutations of case.) Q2: Is 0.50 a good sniff rule weight? I doubt that the rule (".E...B..matroskaB...B....S.g") would pick up non-matroska files. Q3: Is the sniffing rule format documented anywhere? BTW, Haiku's setmime appears to be unimplemented, as of yet. (In case anyone's looking for a nice little project.) /Jonas.