What AVIF is, and why you have one
AVIF is the newest mainstream image format on the web. It is built on AV1, a video codec, and it compresses remarkably well, often producing files half the size of a JPG at the same visual quality.
That efficiency is why sites have adopted it. It is also why you are here, which is that you saved an image, got a .avif, and found your photo viewer, editor, or the form you need to upload to has never heard of it.
This is the same story WebP went through a few years earlier. Browsers adopt new formats quickly because they benefit from the speed. The software on your computer follows years later.