code: function ieInternalDragAndDrop() {
          editor.on('dragstart', function (e) {
            setMceInternalContent(e);
          });
  
          editor.on('drop', function (e) {
            if (!isDefaultPrevented(e)) {
              var internalContent = getMceInternalContent(e);
  
              if (internalContent && internalContent.id != editor.id) {
                e.preventDefault();
  
                var rng = RangeUtils.getCaretRangeFromPoint(e.x, e.y, editor.getDoc());
                selection.setRng(rng);
                insertClipboardContents(internalContent.html, true);
              }
            }
          });
        }
docs: IE cannot set custom contentType's on drag events, and also does not properly drag/drop between
  editors. This uses a special data:text/mce-internal URL to pass data when drag/drop between editors.