Steps to reproduce
On a Pro account, attach video file (pick a small mp4, which would play fine it were to be dragged & dropped to Safari), click on it, the link opens in a new tab.
The video plays on Safari, as it does in Chrome.
Instead of playing the video, Safari shows an “unable to play” icon (https://i.imgur.com/uJ2N3ID.png).
Safari 12.1 on Mojave 10.14.4. This also happens on iOS 12.2.