For platforms that support drag-and-drop, this method is called when the user drags a file onto the window. Drag-and-drop must first be enabled for the window with accept-drop-files.
Under MacOS, when the user double-clicks on a MrEd file or drags a file onto the MrEd icon, the on-drop-file method of the frontmost frame is called (if drag-and-drop is enabled for that frame).