title: p:fileUpload required="true" and custom validator doesn't work
body: <p>Since the <code>required</code> attribute of <code>&lt;p:fileUpload&gt;</code> still doesn't seem to work in PrimeFaces 4.0 final, I have tried to create a custom validator as follows.</p>
  
  <pre><code>@FacesValidator(value="fileUploadValidator")
  public final class FileUploadValidator implements Validator
  {
      @Override
      public void validate(FacesContext fc, UIComponent uic, Object o) 
      throws ValidatorException
      {
          System.out.println("fileUploadValidator called.");
  
          if(!(o instanceof UploadedFile))
          {
              FacesMessage message = new FacesMessage();
              message.setSeverity(FacesMessage.SEVERITY_ERROR);
              message.setSummary("Error");
              message.setDetail("Required");
              throw new ValidatorException(message);      
          }
      }
  }
  </code></pre>
  
  <p>And specified with <code>&lt;p:fileUpload&gt;</code>.</p>
  
  <pre><code>&lt;p:fileUpload mode="advanced" 
                required="true"
                multiple="true"
                allowTypes="/(\.|\/)(gif|jpe?g|png)$/"
                fileUploadListener="#{bean.fileUploadListener}"&gt;
      &lt;f:validator validatorId="fileUploadValidator"/&gt;
  &lt;/p:fileUpload&gt;
  </code></pre>
  
  <p>But the validate method was never invoked. Since I'm displaying images in <code>&lt;p:dataGrid&gt;</code>,  this validation is highly required. Is there a way to validate an empty <code>&lt;p:fileUpload&gt;</code>?</p>