This will make oC create previews for MKVs.
Test file: http://www.auby.no/files/video_tests/h264_720p_hp_5.1_3mbps_vorbis_styled_and_unstyled_subs_suzumiya.mkv
To test this just apply this change and upload the test file, if an preview is generated this change is most likely working as expecting.
'md' => 'text/markdown',
'mdb' => 'application/msaccess',
'mdwn' => 'text/markdown',
+ 'mkv' => 'video/x-matroska',
'mobi' => 'application/x-mobipocket-ebook',
'mov' => 'video/quicktime',
'mp3' => 'audio/mpeg',