我已经完成了单张上传的form表单提交功能，用的就是他们的 js－sdk ，但是现在想多张上传，该如何去做？

var f = new FormData(document.getElementById("testform"));

$.ajax({
  url: 'http://upload.qiniu.com/',  // Different bucket zone has different upload url, you can get right url by the browser error massage when uploading a file with wrong upload url.
  type: 'POST',
  data: f,
  processData: false,
  contentType: false,
  xhr: function(){
    myXhr = $.ajaxSettings.xhr();  
    if(myXhr.upload){
      myXhr.upload.addEventListener('progress',function(e) {
        if (e.lengthComputable) {
          var percent = e.loaded/e.total*100;
          $progress.html('上传：' + e.loaded + "/" + e.total+" bytes. " + percent.toFixed(2) + "%");
        }
      }, false);
    }
    return myXhr;
  },
  success: function(res) {
    var str = '<span>已上传：' + res.key + '</span>';
    if (res.key && res.key.match(/\.(jpg|jpeg|png|gif)$/)) {
      str += '<img src="' + domain + res.key + '"/>';
    }
    $uploadedResult.html(str);
  },
  error: function(res) {  
    $uploadedResult.html('上传失败：' + res.responseText);
  }