title: dropEffect not working when dragging files to Firefox
body: <p>I have a web page that allows files to be dragged from outside the browser and dropped onto a target on my web page.  My application will be uploading a copy of each file, so I set dataTransfer.dropEffect to "copy" so the browser will indicate to the user that this will result in a copy rather than a move.  This is working as expected in Chrome: when dragging a file over my drop target the browser displays a "copy" cursor.  But Firefox seems to ignore the dropEffect and continues to display its default "move" cursor while the files are being dragged over my drop target.  I've done a lot of searching and haven't found any mention of a Firefox problem like this, so I'm probably overlooking some detail in my code.  I've included a stripped-down example that illustrates the problem below.  Thanks in advance if anyone can spot what I'm doing wrong.</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
  &lt;title&gt;Test Stuff&lt;/title&gt;
  
  &lt;style type="text/css"&gt;
  P 
  {
  background-color: #cccccc;
  padding: 10px;
  }
  
  &lt;/style&gt;
  &lt;script type="text/javascript"&gt;
  
  function DocOnLoad() {
  var target = document.getElementById('dropTarget');
  target.addEventListener('dragenter', function (e) {
      e.preventDefault();
      e.dataTransfer.dropEffect = 'copy';
  });
  target.addEventListener('dragover', function (e) {
      e.preventDefault();
      e.dataTransfer.dropEffect = 'copy';
  });
  target.addEventListener('drop', function (e) {
      e.preventDefault();
      var files = e.dataTransfer.files;
      alert(files[0].name);
  });
  }
  
  
  
  &lt;/script&gt;
  
  &lt;/head&gt;
  
  &lt;body onLoad="DocOnLoad()" &gt;
  
  &lt;p id="dropTarget"&gt;Drop target.&lt;/p&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>