I'm trying to add a MP4 video to my media library. It doesn't give any error but when I try to play the video it gives this message inside the video player:
Media error: Format(s) not supported or source(s) not found
The video size is 2MB.
There seems to be a bug in WordPress Core about this:
https://core.trac.wordpress.org/ticket/42874#comment:8
An issue is also opened in the mediaelement GH repo:
https://github.com/mediaelement/mediaelement/issues/2390
The actual problem comes from Safari:
IMPORTANT NOTE for Safari users (Jun 8, 2017)
Since Sierra version, autoplay policies have changed. You may experience an error if you try to execute play programatically or via autoplay attribute with MediaElement, unless muted attribute is specified.
I've also bumped into this issue, but can only reproduce it in Safari, it works on Chrome and Firefox.
Hope this helps!