What currently works: An onBeforeAdd/Update Method that prevents that users with out the required special permission can upload file items. The needed permission are granted on demand only for my automation. I think this a quite reliable solution to prevent the overall upload of files.
But if we still could visibly disable the file picker in the UI it would be the cherry on the top!
I right now ended up in follow double combination:
onAfterAdd/Update prevents server side upload of files (if we don´t come from the right automation context).
onEditStart prevents that user upload something on client side. It simply throws an error message and resets the file property to nothing.
The file picker itself is still visible. I was able to make it "disappear", but it´s empty shell was still clickable. So I simply gave up and rely on the two Methods above.
