It's not uncommon for browsers to save in webp these days, but I haven't seen jfif used. What browser are you using that's doing that? The one thing you can do is control the format your images save in through your settings or I've found an add-on like "Save Webp as..." makes it easy to grab images in different formats. Right now, I'm not sure the plan to support formats like webp or jfif as acceptable files.