code: function(){
              var _body,_text;
              try{
                  var _body = this.__frame.contentWindow.document.body,
                      _text = (_body.innerText||_body.textContent||'').trim();
                  // check result for same domain with upload proxy html
                  if (_text.indexOf(_xflag)>=0||
                      _body.innerHTML.indexOf(_xflag)>=0){
                      // use post message path
                      return;
                  }
              }catch(ex){
                  // ignore if not same domain
                  return;
              }
              this.__onLoadRequest(_text);
          }
docs: same domain upload result check