I made a mistake in my answer.
This concerns the Information Panel, which has a different behavior that the context I had in mind, regular thumbnails in the view.
The information does not cache file rendering, so we don’t use standard thumbnailer, since using our own plugins would save some disk IO.
So here your issue is the opposite it seems, you’d need GitHub - jakar/qt-heif-image-plugin: Qt plugin for HEIF images or GitHub - novomesk/qt-avif-image-plugin: Qt plug-in to allow Qt and KDE based applications to read/write AVIF images. to add support for those formats to qt and KDE apps.
Those might not be available in your distro packages, not sure.
So I can consider this a bug, we probably want to use the standard thumbnailer to read those files, just not store it to cache and remove their file after use.
webp (at least with Qt 6.7+) and PNG are a different issue. The one mentioning audiothumbnail and imagethumbnail: Mention supported mimetypes explicitly (!367) · Merge requests · Network / KIO Extras · GitLab we should probably backport it.