title: Rails file_field_tag not uploading file
body: <p>I have a form:</p>
  
  <pre><code>&lt;%= form_tag :controller =&gt; "foo", :action =&gt; "boo", :multipart =&gt; true do %&gt;
      &lt;%= file_field_tag "foo[bar]" %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>When I submit it, the <code>param foo[bar]</code> gives <code>"foo[bar]"=&gt;"filename.png"</code> instead of an <code>ActionDispatch</code> object.</p>
  
  <p>Anybody have any clues what might be the cause here?</p>
  
  <p>Thanks!</p>