fix MKV datastream mimetpye and delete m4v because it is not exist in mim detect
add mpv to extension list, caz ffmpeg can convert it
Update extensions allowed for upload, DS-COMPOSITE-MODEL allowed extensions.
Merge pull request #1 from jordandukart/7.x-fix-video-mimetype

Update extensions allowed for upload, DS-COMPOSITE-MODEL allowed extensi...
Merge pull request #52 from yqjiang/7.x-fix-video-mimetype

fix MKV datastream mimetpye and delete m4v because it is not exist in mi...