Embedded video / audio in Markdown is not playable in Safari and IE

From !18354 (comment 230276265).

This may be related to https://stackoverflow.com/questions/27712778/video-plays-in-other-browsers-but-not-safari, but I see that we support byte range requests so I'm not sure what the problem is.

Also, the same happens when we embed audio / video in the blob viewer.