Hi Sam,
Thanks a lot for your answer.
My application is not worth writing a Xcode or Swift plugin and, even if it was, I don’t have a sufficient knowledge to do it.
The app is not published yet, so the easiest is to remove the ability to drop on the application icon. So the future customers won’t complain about regression when they upgrade from El Capitan to Sierra.
What is surprising is that I can drag/drop from Photos on the application windows, just not on the icon.
What is also strange is that this doesn’t work anymore with Mail which is an Apple application.
I’ll search and monitor stack overflow to see if other developers have the same issue with Xcode/Swift apps.
Thanks anyway.