Aktivieren von Miniaturansichten für benutzerdefinierten Beitragstyp

Ich schreibe einen benutzerdefinierten Post-Typ als Plugin und versuche herauszufinden, wie ich Post-Thumbnails in der Plugin-Dokumentation/-Vorlage aktivieren kann, anstatt sie in der functions.php zu aktivieren. Ich habe versucht, den folgenden Code in custom-post-plugin.php einzufügen und es scheint nicht zu funktionieren. Ich muss den Code noch in die functions.php schreiben, um die Miniaturansichten für meinen benutzerdefinierten Beitrag zu aktivieren

/ * ADD THUMBNAIL FOR POST
 * ------------------------------------------------------------------------ */
add_theme_support( 'post-thumbnails', array( 'post', 'custom-post' ) );

Beispielcode unten

<?php
/*
Plugin Name: Example Custom Post Plugin
Description: Example post plugin for News and Press custom post
*/
?>

<?php
/* ------------------------------------------------------------------------ *
 * EXAMPLE CUSTOM POST
  ------------------------------------------------------------------------ */

function example () {

// Set UI labels for Custom Post Type
    $labels = array(
        'name'                => _x( 'Example', 'Post Type General Name', 'twentythirteen' ),
        'singular_name'       => _x( 'Example', 'Post Type Singular Name', 'twentythirteen' ),
        'menu_name'           => __( 'Example', 'twentythirteen' ),
        'parent_item_colon'   => __( 'Parent Example', 'twentythirteen' ),
        'all_items'           => __( 'All Post', 'twentythirteen' ),
        'view_item'           => __( 'View Post', 'twentythirteen' ),
        'add_new_item'        => __( 'Add New Post', 'twentythirteen' ),
        'add_new'             => __( 'Add Post', 'twentythirteen' ),
        'edit_item'           => __( 'Edit Post', 'twentythirteen' ),
        'update_item'         => __( 'Update Post', 'twentythirteen' ),
        'search_items'        => __( 'Search Post', 'twentythirteen' ),
        'not_found'           => __( 'Not Found', 'twentythirteen' ),
        'not_found_in_trash'  => __( 'Not found in Trash', 'twentythirteen' ),
    );

// Set other options for Custom Post Type

    $args = array(
        'label'               => __( 'Example', 'twentythirteen' ),
        'description'         => __( 'Example', 'twentythirteen' ),
        'labels'              => $labels,
        // Features this CPT supports in Post Editor
        'supports'            => array( 'title', 'editor', 'excerpt', 'author', 'thumbnail', 'comments', 'revisions', 'custom-fields', ),
        // You can associate this CPT with a taxonomy or custom taxonomy.
        'taxonomies'          => array('category', 'post_tag'),
        /* A hierarchical CPT is like Pages and can have
        * Parent and child items. A non-hierarchical CPT
        * is like Posts.
        */
        'hierarchical'        => false,
        'public'              => true,
        'show_ui'             => true,
        'show_in_menu'        => true,
        'show_in_nav_menus'   => true,
        'show_in_admin_bar'   => true,
        'menu_position'       => 4,
        'can_export'          => true,
        'has_archive'         => true,
        'exclude_from_search' => false,
        'publicly_queryable'  => true,
        'capability_type'     => 'page',
    );

    // Registering your Custom Post Type
    register_post_type( 'example', $args );
}

/* Hook into the 'init' action so that the function
* Containing our post type registration is not
* unnecessarily executed.
*/

add_action( 'init', 'example', 'example', 1 );

add_action( 'after_setup_theme', 'example_featured_image');
function example_featured_image() {
        add_theme_support( 'post-thumbnails', array( 'post', 'page', 'production_firm', 'example' ) );
}