Latest update: I got the rendering of AVIF files working with
GraphicsMagick:

  * Made sure that GraphicsMagick properly reports its list of supported
    file formats.  By doing so I found what I believe to be a [bug in
    Arch][1].

  * Explicitly specified GraphicsMagick as converter, although according
    to documentation it should be chosen automatically if not set:

         '(image-converter 'graphicsmagick)

  * As before, I specified `avif` as an extension of files to render in
    `image-mode` upon loading:

        '(image-file-name-extensions
          '("png" "jpeg" "jpg" "gif" "tiff" "tif" "xbm" "xpm" "pbm" 
            "pgm" "ppm" "pnm" "svg" "avif"))

    However, that does *not* work.  AVIF files are opened in
    `fundamental-mode`, and I have to do:

        M-x image-mode

Now, I would like to try if setting `image-file-name-regexps` can solve
the issue.  However, when I add a regular expression such as simply the
four characters “avif”, then customize reports an error message:

    image-file-name-regexp: Wrong type argument: characterp, "avif"

*Why is that?  How do I properly specify a regular expression here?*

See full configuration attached.

(About raw files: GraphicsMagick does support ARW files, albeit
rendering is slow.  Via so called delegates it can be extended to
support additional file formats, and maybe I can tell it to use the
preview embedded in an ARW in order to speed things up.)

[1]: https://bugs.archlinux.org/task/77182