file_save_upload custom validators?

Is it possible to create custom validators for file_save_upload?

From http://drupal.org/node/286374 I think it's possible. I'm twiking some code to make the profile picture required in registration form and since '#required' doesn't work for upload fields (see: http://drupal.org/node/42762 ), I thought that maybe creating a custom validation would do the trick.

I've started from the code of http://drupal.org/project/reg_with_pic which adds the upload picture field on registration form.

I cannot find documentation on how to implement custom validators. For now I have this code that does not work:

  $validators = array(
    'reg_with_pic_file_required' => array(),
    'file_validate_is_image' => array(),
    'file_validate_image_resolution' => array(variable_get('user_picture_dimensions', '85x85')),
    'file_validate_size' => array(variable_get('user_picture_file_size', '30') * 1024), 
  );
  $file = file_save_upload('picture_upload_register', $validators);

and

  function reg_with_pic_file_required(&$file){
    $errors = array();
    if(!isset($file->filepath)){
      $errors[] = t('Image is required. Please upload an image');
    }
    return $errors;
  }

It never calls the reg_with_pic_file_required function. I've tried putting some drupal_set_message's but they never show so I assume that this function is never called. Please note that the checking I do on the "if" is temporal, I'll look into a proper checking when I know the function gets called.

Anyone can tell me what am I doing wrong and how can I solve this?