How do I get the error message for the extension validation rule to display?

Hi,

I am new to CakePHP and I am having trouble getting the extension error message to display. The error message for the email rule displays fine when someone enters an improperly formatted email and I tried other examples of Cake's built in validation, they all seem to work fine except for the extension validation.  Can someone please tell me what I am doing wrong.

Here are the validation rules in my model:
  var $validate = array(
    'imageupload' => array(
        'rule' => array('extension', array('gif', 'jpeg', 'png', 'jpg'),
        'message' => 'Please supply a valid image.'
        )
        )
        ,
'email'=>array(
'rule'=>'email',
'message'=>'Please enter a proper email'
        )
    );


Here is the snippet code in the Controller:
   function add(){
        if (!empty($this->data)) {
           
            if($this->Student->save($this->data)){
               
                $this->flash('Success','/add');
            }
        }
    }

Finally the snippet code in my view:
<?PHP
echo $form->create('Student',array('type' => 'file'));
echo $form->input('email');
echo $form->file('imageupload');
echo $form->end('Add');
?>

Again, the display message for the email displays correctly so I know that the validation is working at least somewhat.  The validation for the file upload seems to operate correctly. If I try to upload a file with a .DOC extension the "save" fails except that it does not display the error message at all.

Please help.... what am I doing wrong?