我正在尝试发送一个multipart/form-data内容类型请求:varxhr=newXMLHttpRequest();xhr.onreadystatechange=function(){if(xhr.readyState==4){alert(xhr.responseText);}}xhr.open("POST",url,true);xhr.setRequestHeader("Content-Type","multipart/form-data;boundary=---------------------------275932272513031");xhr.send('-----