With the only working solution for the heic files (that is the Opus heic plugin disabled!) I still have the thumbnails and the preview handler working correctly.
Now, is it possible to view these files in the Image viewer as well?
Normally the Image viewer doesn’t run when you double click on a heic file; so the first step would be to run it. But when I first open a jpg file in it, then drag an drop the heic file, it shows the latter as a binary file with the text viewer plugin instead.