title: How to set file extension in Wicket, FileUploadField?
body: <p>I have a FileUploadField component, and I want to upload only <code>*.css</code> or <code>*.html</code> files with that. But now, I can upload any type of the files. How could I set the extensions of the files to that component? My code peace:</p>
  
  <pre><code>FileUploadField ret = new FileUploadField("uploadField");
  ret.setRequired(true);
  form.add(fileUploadField);
  </code></pre>
  
  <p>Thank you!</p>