When an MPEG-4 video is served up with a "Content-Disposition: attachment" HTTP header it won't work as the <source> of an HTML5 <video> tag in either the iOS or macOS versions of Safari. Instead you just get a blank white box with a play icon that does nothing when you press it.
On a Mac you can right-click it and choose "Open Video in New Window" which opens a new video and then immediately downloads the video (correct behavior, because of the "Content-Disposition: attachment").
Safari users on iOS can't do anything to see the video. (And switching to another browser doesn't help, due to Apple's prohibition on non-Webkit browsers on the iOS App Store.)
It's not clear to me whether Safari is acting correctly or not. In my opinion Apple should change Safari to ignore Content-Disposition for videos displayed using <video> but until then the fix is to change the Content-Disposition on the server side.
Here's an example of a video that won't play in Safari: https://community.glowforge.com/t/pre-release-mothers-day-part-2-edge-lit-led-sign/7440
And the HTTP headers on the embedded video:
$ curl -I https://cdn-business2.discourse.org/uploads/glowforge/original/3X/7/f/7f89d335777d58b197db6513ed918f1bc65f0245.mp4
HTTP/1.1 200 OK
Date: Mon, 08 May 2017 23:06:32 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
)
