Просто добавьте что-то подобное в функцию add () вашего контроллера.
// File functions
if ($this->data['MODELNAME']['FILENAME']) {
$file = new File($this->data['MODELNAME']['FILENAME']['tmp_name']);
$ext = $this->data['MODELNAME']['FILENAME']['name'];
$point = strrpos($ext,'.');
$ext = substr($ext,$point+1,(strlen($ext)-$point));
/*if ($ext != 'jpg' && $ext != 'jpeg' && $ext != 'gif' && $ext != 'png') {
$this->Session->setFlash('You may only upload image files.');
$this->render();
} else {*/
$filename = 'filename.'.$ext;
$data = $file->read();
$file->close();
$file = new File(WWW_ROOT.'/img/'.$filename,true);
$file->write($data);
$file->close();
//}
}