Can you not just use the accept extension?

I suppose I could but this seems like a reasonable PR. AVIF and WebP are now common on the web so where PNG, JPEG and GIF are accepted AVIF and WebP are expected to be accepted too.

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Someone can correct me if I'm wrong, but after reviewing the Git history I don't think there is precedent for updating existing additional rules. IMO these changes constitute a breaking change and would therefore need to wait until v2. I think you're better off just doing what I suggested, or define your own rule...