跨域下载，a标签download设置下载文件名失效的问题

download (fileUrl, fileName, ext) {
    
    
      var x = new XMLHttpRequest();
      x.open("GET", fileUrl, true);
      x.responseType = 'blob';
      x.onload = function (e) {
    
    
        var url = window.URL.createObjectURL(x.response)
        var a = document.createElement('a');
        a.href = url
        a.download = fileName + '.' + ext;
        if (ext == 'pdf') {
    
    
          a.target = '_blank'
          a.click()
        }
        a.click()
      }
      x.send();
      }