title: Yii: rules 'allowEmpty'=>true still raise 'cannot be blank'
body: <p>I have a problem with CActiveRecord.rules</p>
  
  <pre>
  public function rules(){
      return array(
          array('photo_path', 'required', 'on'=>'insert'),
          array('photo_path', 'file', 'types'=>'jpg, gif, png', 'allowEmpty'=>true),
      );
  }
  </pre>
  
  <p>Photo_path should be required only when the model is created, in update view it can be empty.</p>
  
  <p>I dont know what happends, but one hour ago it works, but now it doesn't.
  When I choose file I get 'photo path cannot be blank'</p>
  
  <p>Please help :) </p>