I just upgraded from Moodle 3.1 to 3.3 and .mp4 files that worked on Internet Explorer and Chrome display an unsupported media type. These same files work with Firefox. flv files work fine.
Any suggestions would be greatly appreciated.
Take a look at your media filters - that is probably where the problem is.
I have the media filter set to HTML5 video as it was prior to upgrade. The thing that puzzles me is that the .mp4 files display fine in both Firefox and Edge but fail to display in Chrome and IE as they did previously.
It seems like I had to switch at some point. I have the VideoJS player at the top of the list - you might try that.