functions.php

<?php
  add_filter('pre_get_posts', 'klausk_category_post_types');
  function klausk_category_post_types($query) {
    if((is_category() && $query->is_main_query()) {
      $post_type = get_query_var('post_type');
      if($post_type)
        $post_type = $post_type;
      else
        $post_type = array('post','resort');
      $query->set('post_type',$post_type);
    }
    return $query;
  }
?>