Requested feature overview description.
avif is an image format that https://skeb.jp/ started using.
Japanese artists use that site to handle commissions. Sometimes the artist or the commissioner upload the commission on their twitter accounts but some of them are only available on skeb. Example image with the avif ext https://skeb.jp/@N_ioNion/works/30

Why would it be useful?
the original avif image can be uploaded instead of having to convert the image to the currently supported file ext

What part(s) of the site page(s) are affected?
Upload.

error: ActiveRecord::RecordInvalid - Validation failed: File ext image/avif is invalid (only JPEG, PNG, GIF, and WebM files are allowed

Can the avif ext images be converted to PNG to be able to be uploaded until avif gets support for uploading?

How many of them are using AVIF? Are they uploading lossless AVIF?

lance_armstrong said:
Are they uploading lossless AVIF?

I don't know about that

lance_armstrong said:
How many of them are using AVIF?

All the images on that site are now AVIF. I think skeb started converting all the images to AVIF since march or late february. The last image that was available only on skeb I uploaded on february was a jpg https://e621.net/posts/3163427. Now if you download it from the skeb source it downloads as an AVIF
https://skeb.jp/@_hysk_/works/8
https://skeb.imgix.net/uploads/origins/2067e5c7-8b48-491a-a08f-8c8c95d0f787?bg=%23fff&auto=format&txtfont=bold&txtshad=70&txtclr=BFFFFFFF&txtalign=middle%2Ccenter&txtsize=150&txt=SAMPLE&w=800&s=22e1dd12c49c72d7c983171e1b10f365

You can download a plugin to get AVIF to display on Windows, is there a way that would work for the site too?

Chrome, Firefox and Opera support it in both desktop and mobile versions.
Source: https://caniuse.com/avif
But maybe YAGNI .