title: Cannot get dropped folder information in FIrefox 48
body: <p>I cannot get dropped folder information, in Firefox 48, Windows 10.
  <code>event.dataTransfer.files.length</code> is 0, when I drop folder.
  It works fine when I drop file.</p>
  
  <p>If you know a recommended method to get dropped folder information, I wish to know it.</p>
  
  <p>You can reproduce it in <a href="https://jsfiddle.net/0GiS0/4ZYq3/" rel="nofollow">https://jsfiddle.net/0GiS0/4ZYq3/</a>.</p>