title: input type="file" is not opening the file browser from Chrome Extension
body: <p>I'm not sure if it's a bug or something simple that I just missed...
  Here is a simple test case. The index.html is:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
    &lt;head&gt;
      &lt;meta charset="utf-8" /&gt;
      &lt;title&gt;Test file input field&lt;/title&gt;
    &lt;/head&gt;
    &lt;body&gt;
      &lt;label for="file"&gt;File&lt;/label&gt;
      &lt;input type="file" name="file" id="file"/&gt; 
    &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>The manifest is calling it inside a popup:</p>
  
  <pre><code>"popup": "index.html"
  </code></pre>
  
  <p>I was expecting to get the 'file browser' after clicking the 'file' button but it is vanish after 0.5sec. 
  (It's on Mac 10.6.8 and Chrome 14.0.835.186)</p>
  
  <p>Any idea?</p>
  
  <p>Thanks...</p>