Converting Affinity files

Affinity photo files (*.afphoto) are shown without problems in the DO listers. But I could not find a context menu to convert those pictures to another format (jpg, png,...). Did I miss something?

You might be able to, with the files selected, use Tools > Convert Images to do it.

I don't think it's a format Opus (or any Opus plugins) knows anything about. If you can see thumbnails for it, they're probably coming from something that has installed a system-wide thumbnailer for the format.