title: Ajax response is a file download
body: <p>I'm sending a XMLHTTPRequest to a server endpoint using Polymer's <a href="https://elements.polymer-project.org/elements/iron-ajax" rel="nofollow">iron-ajax</a> element:</p>
  
  <pre><code> &lt;iron-ajax
    id="ajax"
    method="POST"
    url="/export/"
    params=''
    handle-as="json"
    on-response="handleResponse"
  &lt;/iron-ajax&gt;
  </code></pre>
  
  <p>My Koa/Express-server responds with a read stream like this:</p>
  
  <pre><code>router.post('/export' , function*(){
  
    var file = __dirname + '/test.zip';
    var filename = path.basename(file);
    var mimetype = mime.lookup(file);
  
    this.set('Content-disposition', 'attachment; filename=' + filename);
    this.set('Content-type', mimetype);
    this.body = fs.createReadStream(file);
  })
  </code></pre>
  
  <p><strong>How do I initiate the download in <code>handleResponse()</code>?</strong>
  Ideally I don't want to handle the response at all and directly initiate the download.</p>
  
  <p>The response headers look (as expected) like this:</p>
  
  <pre><code>Content-disposition: attachment; filename=test.zip
  Connection: keep-alive
  Transfer-Encoding: chunked
  Content-type: application/zip
  </code></pre>