Simple node module - attach a required file?

Hi,
I'm trying to make a module with a specific handling of a file, so I do not want to use upload module, thanks.
I've resumed my problem in a toto module, which is below. I've tried several things, without success. I have always a "toto file field is required.".

<?php
function toto_node_info() {
  return array(
   'toto' => array('name' => t('toto'), 'base' => 'toto')
  );
}
function toto_menu($may_cache) {
  $items = array();
  if ($may_cache) {
    $items[] = array('path' => 'node/add/toto', 'title' => t('toto'),
      'access' => user_access('test toto'));
  }
  return $items;
}
function toto_form(&$node, &$param) {
  $form['totofile'] = array(
   '#type' => 'file',
   '#title' => t('toto file'),
   '#required' => TRUE,
   '#weight' => 0);
  return array_merge($form, filter_form($node->format));
}
function toto_form_alter($form_id, &$form) {
  $form['#attributes'] = array('enctype' => 'multipart/form-data');
}
function toto_validate(&$node) {
  if ($file = file_check_upload('totofile')) {
  }
}
?>

Thank you.