AVIF support

This is a community project and I expect they would be happy with your pull request to implement this., /Since you have a need for this, it might be a good first PR for you..

Looking at a high level I would expect it just needs to be added as recognized & mapped to the proper MIME type.

It's a bit more complex because AVIF was not supported in the image upload classes of CI (resulting in a 500 error). There are MIME types, the upload process, and other directives in the core that need to be modified.

You have a vested interest and a need for this. The CI dev team is very small and busy working on other issues affecting more users.