title: Input accept="image/png" is not working in Firefox
body: <p><a href="http://jsbin.com/EHAJEToF/1/edit" rel="noreferrer">jsbin</a></p>
  
  <pre><code>&lt;input type="file" accept="image/png"&gt;
  </code></pre>
  
  <p>It's expected that the file dialog accepts png files only. But <code>accept="image/png"</code> does not work on Firefox. How can I make it?
  P.S. It works in Chrome.</p>