Deletar anexos automaticamente ao deletar um post no WordPress: https://www.marcelotorresweb.com/deletar-anexos-automaticamente-ao-deletar-um-post-no-wordpress/

delete-associated-post.php

<?php
function delete_associated_post( $post_id ) {

    if( get_post_type( $post_id ) === 'books' ){

        $medias = get_children( array(
            'post_parent' => $post_id,
            'post_type'   => 'attachment'
        ) );

        if( $medias ){
            foreach( $medias as $media ) {
                wp_delete_attachment( $media->ID );
            }
        }

    }
}
add_action( 'before_delete_post', 'delete_associated_post' );