problem uploading files in altered form

hi, this is my first try with form_alter hook i would like to know if i hava some kind of newbe mistake...
the form is showing the upload file in the form, but it seams to be empty at submit time.

this is my code :

function taharon_form_alter(&$form, $form_state, $form_id) {
   
  switch ($form_id)
  {
    case 'story_node_form':
   	  
	   $form['imagen'] = array(
			'#type' => 'file',
			'#title' => 'imagen para el slideshow',
		    			
		
		);
	   
		$form['#attributes'] = array('enctype' => "multipart/form-data");
		$form['#validate'][] = '_validar_imagen_validate';
		break;
   }
}
 

  function _validar_imagen_validate($form, &$form_state) {
  
  $file = file_save_upload('imagen');
  var_dump($form_state);
  
 } 

Any help?