What file type
image/avif

What application is using it
Not too many yet.

Please attach a sample file here (or a direct external link to one)
avif sample file

Browser support isn't full yet but there's https://github.com/Kagami/avif.js for instance which works just fine.

AVIF-compressed files can also be embedded inside HEIF containers so support might also be added as part of #4