But anyway, if the problem is that the json file is opened in a new tab instead of downloading, can't you just encode it with a non-text media type in the data-uri so the browser doesn't understand it?
E.g, data:application/octet-stream instead of data:image/png which is what you claim works right now
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.
I always felt like data: urls could obtain a lot more utility if disposition and filename attributes were added to it, but... uhm, browser makers don't seem to agree, and the syntax would make it difficult to add features like that whilst staying backwards compatible.
I can confirm that, most of my time spent for browser support is on IE11 and Safari. Everything in Edge, Firefox and Chrome generally works fine.