转换流

    var xhr = new XMLHttpRequest();
    xhr.open("GET", url, true);
    xhr.responseType = "blob";
    xhr.onload = function () {
      if (xhr.status === 200) {
        cb(xhr.response);
      }
    };
    xhr.send();
  };
  
  
  ## 保存
    const saveAs = (blob, filename) => {
    if (window.navigator.msSaveOrOpenBlob) {
      navigator.msSaveBlob(blob, filename);
    } else {
      var link = document.createElement("a");
      var body = document.querySelector("body");

      link.href = window.URL.createObjectURL(blob);
      link.download = filename;

      // fix Firefox
      link.style.display = "none";
      body.appendChild(link);

      link.click();
      body.removeChild(link);

      window.URL.revokeObjectURL(link.href);
    }
  };
  
  
  ### 使用
     getBlob(file.url, function (blob) {
      saveAs(blob, url[url.length - 1]);
    });