Uh you can definitely set the mime type in data uris. It's like right there as the first part of a data uri...

Download attribute let's you choose a filename. But giving up on that for safari sounds like a decent tradeoff to me (with the current approach you end up with a nonsense png extension in the save file anyway).

Testing with a quick demo: https://codepen.io/anon/pen/wYpBry?editors=1010, it seems like mobile safari will show it as unknown.txt but have options to save to files/dropbox etc. Kind of complicated but probably still less confusing than a png file.