The solution to this problem is so blissfully easy, it really does seem too good to be true: You can just rename the file. No image conversion necessary. Do not open a jfif in MS Paint or Photoshop and bother resaving it. Just click on the file name, press backspace four times, type jpg (or jpeg), and hit Enter. You now have a perfectly usable image. The two in the screenshot above are the exact same files, just with one quickly renamed.