title: jQuery ajax, send form with input type file and Json
body: <p>I have a form with several inputs and other field.</p>
  
  <p>I have a save button, and when I click, I send the form with ajax with jQuery :</p>
  
  <pre><code>$.ajax({
          type:       "POST",
          dataType:   "json",
          url:        $('#ajaxUrl').val(),
          data:       "action=save&amp;" + form.serialize()
  });
  </code></pre>
  
  <p>So, when I have only simple input like text, select etc.. it's ok. But if I have an input type file, I can't retrieve my file. $_FILES is always empty.</p>
  
  <p>How can I do that as simply as possible ?</p>
  
  <p>Edit : I don't want to use a plugin :)</p>