the_content hook differentiate by post type

add_filter('the_content', 'custom_filter');

 function custom_filter($content) {
  global $post;

  if($post->post_type == 'post') {
    $var = 'This is a post';
  } elseif($post->post_type == 'page') {
    $var = 'This is a page';
  } else {
    $var = 'This is some custom post type, named '.$post->post_type;
  }
  return $content.$var;
 }