title: Blobstore upload + Ajax/Alternative
body: <p>The following code works perfectly. My only concern is that I wanna convert below to AJAX/alternative, so that it doesn't need to refresh the whole page to submit this request.</p>
  
  <p>If possible, to also include loading progress bar etc.</p>
  
  <pre><code>&lt;form action="{{ upload_url }}" method="POST" enctype="multipart/form-data"&gt;
  
          Upload File: &lt;input type="file" name="file"&gt; &lt;br&gt; 
          &lt;input type="submit" name="submit" value="Submit"&gt; 
  
          &lt;input type="hidden" name="data1" value="{{ data1 }}"&gt;
          &lt;input type="hidden" name="data1" value="{{ data2 }}"&gt;
  
  &lt;/form&gt;
  </code></pre>