title: Submitting a file with jQuery.ajax gives TypeError
body: <p>I am trying to submit a file from a form using jQuery's <code>ajax</code> method:</p>
  
  <pre><code>var ofile=document.getElementById('image').files[0];
  var formdata = new FormData();
  formdata.append("image",ofile);
  
  $.ajax({
      url:'elements/save_elements',
      data:formdata,
      type:'POST'
  });
  </code></pre>
  
  <p>This results in the error <code>TypeError: 'append' called on an object that does not implement interface FormData</code>.</p>
  
  <p>What causes this error? It doesn't happen on the actual <code>formdata.append</code>, but inside jQuery.</p>