Sending Pictures as AVIF files

I’ve got the same problem. Downloaded images from ruggable website on my iphone15. The format is AVIF. I tried forwarding via message to my BF’s iPhone15. He gets thumbnail versions. And it doesn’t open for him. But in my photo library it’s full size

If you believe that "etc" includes macOS, then sad to report, no, it doesn't.

I tried to open an AVIF file on macOS in: Preview, Safari, Chrome, Brave, and Discord, and none of them would show it. My friend with a Windows machine did a screen share with me and showed me that he had no such problem. Something is messed up here.

The other person is an Android user or iPhone user is not clear from your post.

It’s perfectly clear.

I would block, avif files if possible on IOS

All recent versions of Windows, Android, Linux, iOS , etc. support AVIF.

If you can open and see the image, the issue is on the receivers end.