title: wicket issue on nested forms when use FileUpload
body: <p>i wrote a panel in wicket that let me upload a file and it totally works.<br>
  but when i place that panel in an other form (<code>wicket:id="outerForm"</code>), and upload a file, it throws an exception from (<code>component id= outerForm</code>):<br></p>
  
  <p><code> java.lang.IllegalStateException: ServletRequest does not contain multipart content. One possible solution is to explicitly call Form.setMultipart(true), Wicket tries its best to auto-detect multipart forms but there are certain situation where it cannot.</code></p>
  
  <p>but for <code>outerform</code> I've call <code>form.setMultipart(true)</code></p>
  
  <p>does anyone know how to fix it?<br>
  i use wicket 1.4.21</p>
  
  <p>thanks</p>