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.
I didn't say anything about mime type. I said disposition (which is what you would use to force a file to download, instead of display in browser, using the http protocol), and filename. If you set mimetype to octet, mobile safari, since it does not support the download attribute, cannot name it anything sensible or choose an appropriate file extension.
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.
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.