No, it works fine, as you can see here. This implies there’s a problem with the install on their end.
Unless you can repro a problem on try, but I’m not seeing one. Here’s a screenshot of me sending a PM to myself with that sample video clip on try.discourse.org
Raw post source is
https://discourse-cdn.freetls.fastly.net/try/uploads/default/original/1X/17735b4721faa7d59e1fd8a2300135a3f04c9ea6.mp4