Add mimetype for MKV videos

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.
Merge pull request #7806 from owncloud/add-matroska-video

Add mimetype for MKV videos