Don't limit file viewer's previews to 8k hdr images, and also show previews for EXR

When selecting a HDRI using the file viewer, it's necessary to turn on thumbnail mode so I can see what the HDRI looks like. This is currenlty limited to 8k for images with a HDR extension, and for EXR files, it doesn't show a thumnail at all.

It would be great to have both of those limitations removed, otherwise I have to always download a small resolution HDR so I get a preview of the larger resolution one beside it, and for EXR, I have to save a smaller copy as a jpg.