Discourse has had this issue with videos in Safari for a while. Presumably the @discourse people will fix it at some point. I think the problem is that the server sets the Content-Disposition to “attachment”:
$ curl -I /uploads/db6859/original/3X/7/f/7f89d335777d58b197db6513ed918f1bc65f0245.mp4
HTTP/1.1 200 OK
Date: Mon, 01 May 2017 22:07:36 GMT
Content-Type: video/mp4
Content-Length: 5107512
Connection: keep-alive
Last-Modified: Mon, 01 May 2017 17:37:16 GMT
Content-Disposition: attachment; filename="Flashing.mp4"
Cache-Control: private
Accept-Ranges: bytes
Discourse-Proxy-ID: app-router-tiehunter02
Strict-Transport-Security: max-age=31415926
Server: NetDNA-cache/2.2
X-Cache: MISS
Safari users can right-click on the blank spot and choose “Open Video in New Window”; this will cause the video to download (due to the “Content-Disposition: attachment”) and the downloaded video will play fine. I don’t know why Safari refuses to play embedded videos when they’re marked as attachments, but it’s been an issue for years.