DragAndDrop.visualMode  

function OnGUI ()
{
var eventType = Event.current.type;
if (eventType == EventType.DragUpdated || eventType == EventType.DragPerform)
{
// Show a copy icon on the drag
DragAndDrop.visualMode = DragAndDropVisualMode.Copy;

if (eventType == EventType.DragPerform)
DragAndDrop.AcceptDrag();

Event.current.Use();
}
}