Dragging image to default material issue

Dragging an image from the project palette to a material in the scene graph will add that image as a map to the material. However, if the material is the Default material, this will instead create a rectangle as a child to the object and add the image as a diffuse map to the rectangle.

This might be expected behavior but can be confusing. Maybe disabling the option to drop the image on the material would be better in these cases? I would still be possible to drop is as a child to the object.