Upstreaming this from https://phabricator.wikimedia.org/T75990
OGG video files and OGG audio files can both be "application/ogg".
Phabricator has "application/ogg: true" set for "files.audio-mime-types" by default. Same for "files.viewable-mime-types".
Hence in tasks that refer to an OGG video file, there is an embedded audio-only preview shown (and many screencast video files do not have sound which makes people think that something's really borken).
Assuming that content sniffing for OGG files is a bit heavy, I'd propose to set
"application/ogg" : false
by default under "files.audio-mime-types" as it can be both video and audio.
After doing so, there is no file preview in the task anymore and when clicking the link to the video file, the video is now flawlessly played in Firefox 33.
Before that I had to go to the File page, right-clicking on "View File", choose "Save as", and open the file in the native video player of my OS to watch it.