title: Prompt file download with XMLHttpRequest
body: <p>I'm aware that jQuery's ajax method cannot handle downloads, and I do not want to add a jQuery plugin to do this.</p>
  
  <p>I want to know how to send POST data with XMLHttpRequest to download a file.</p>
  
  <p>Here's what I've tried:</p>
  
  <pre><code>var postData = new FormData();
  postData.append('cells', JSON.stringify(output));
  
  var xhr = new XMLHttpRequest();
  xhr.open('POST', '/export/', true);
  xhr.setRequestHeader("X-CSRFToken", csrftoken);
  xhr.responseType = 'arraybuffer';
  xhr.onload = function (e) {
      console.log(e);
      console.log(xhr);
  }
  xhr.send(postData);
  </code></pre>
  
  <p>I'm working with Django, and the file appears to be sending back to the client successfully.   In the network tab in Chrome, I can see gibberish in the preview tab (which I expect).  But I want to send back a zip file, not a text representation of the zip file.  Here's the Django back end:</p>
  
  <pre><code>wrapper = FileWrapper(tmp_file)
  response = HttpResponse(wrapper, content_type='application/zip')
  response['Content-Disposition'] = "attachment; filename=export.zip"
  response['Content-Length'] = tmp_file.tell()
  return response
  </code></pre>
  
  <p>I've searched this for hours now without finding a proper example on how to do this with XMLHttpRequests.  I don't want to create a proper html form with a POST action because the form data is rather large, and dynamically created.</p>
  
  <p>Is there something wrong with the above code?  Something I'm missing?  I just don't know how to actually send the data to the client as a download.</p>