As far as i understand the ExternalObjectsDropped event handler is dealing with native GOJS parts that where dropped.
Is it possible to drag and drop an external object onto a diagram which is NOT a native gojs part ?
Yes, but you need to implement it yourself. For example: https://gojs.net/latest/samples/htmlDragDrop.html.
Or if you are old-fashioned: https://gojs.net/latest/samples/jQueryDragDrop.html