Cannot edit old thumbnail image, required to upload a 'rendering' image
The old image that could be uploaded for symbolizing download files ('thumbnail') has been replaced by a different logic. This means that the old thumbnail is no longer accessible for editing.
It's a bit confusing, that it shows an image for my extension, which I cannot edit or exchange. Also, I cannot save the extension, even though there already is a thumbnail, without uploading a 'rendering' image.
I haven't tested what happens with the rendering image - if it would replace the old thumbnail in the gallery or not.
It's just a quirk, and probably doesn't concern many users. Maybe copying the thumbnails into the rendering field could help? Or would that break something?