I’ve tried to follow this documentation to drag a Windows .ico file and populate the Images and Masks for the App icon:
"You can also drag an ICNS or ICO file onto the window background (instead of an image area) to load and populate all the image sizes at one time. If you add an icon that has both the image and the mask (such as with most PNG files) then both the image and mask are added. "
This just will NOT work. No attempt to drag will result in a drop.