为了清楚起见：借用this answer, 将以下内容添加到主题functions.php:

function wpse_188427_delete_post_media( $post_id ) {
    $attachments = get_posts(
        array(
            \'post_type\'      => \'attachment\',
            \'posts_per_page\' => -1,
            \'post_status\'    => \'any\',
            \'post_parent\'    => $post_id,
        )
    );
    
    foreach ( $attachments as $attachment ) {
        wp_delete_attachment( $attachment->ID );
    }
}

add_action( \'before_delete_post\', \'wpse_188427_delete_post_media\' );

// Uncomment the following line if you also want to delete media when the post is trashed
// add_action( \'wp_trash_post\', \'wpse_188427_delete_post_media\' );