I don't see any mention of them so throwing it out there since browser support is coming faster than any other formats that ever existed.
When you view a jxl or avif it currently says: Binary file not shown.
For now a setting in the app.ini file would make sense I guess to allow them to be previews for whoever has it enabled.

All major browsers can now be enabled / already ship with it enabled.
Jpeg XL thumbs (.jxl)
Mime type: image/jxl
https://caniuse.com/jpegxl

AVIF (AV1's image from the Alliance for Open Media)
Mime type: image/avif
https://caniuse.com/avif