Display a custom post type's media library inline on the WordPress edit page screen

wordpress_custom_post_gallery.php

<?php
 
  define('MY_POST_TYPE', 'my');
  define('MY_POST_SLUG', 'gallery');

  function my_register_post_type () {
  	$args = array (
  		'label' => 'Gallery',
  		'supports' => array( 'title', 'excerpt' ),
  		'register_meta_box_cb' => 'my_meta_box_cb',
  		'show_ui' => true,
  		'query_var' => true
  	);
  	register_post_type( MY_POST_TYPE , $args );
  }
  add_action( 'init', 'my_register_post_type' );

  function my_meta_box_cb () {
  	add_meta_box( MY_POST_TYPE . '_details' , 'Media Library', 'my_meta_box_details', MY_POST_TYPE, 'normal', 'high' );
  }

  function my_meta_box_details () {
  	global $post;
    $post_ID = $post->ID; // global used by get_upload_iframe_src
  	printf( "<iframe frameborder='0' src=' %s ' style='width: 100%%; height: 400px;'> </iframe>", get_upload_iframe_src('media') );
  }

Sounds great. Where do I paste this code?