Temanızın functions.php dosyasına ekleyerek dener misiniz?

add_action( 'before_delete_post', 'wps_remove_attachment_with_post', 10 );
function wps_remove_attachment_with_post($post_id)
{
if(has_post_thumbnail( $post_id ))
{
$attachment_id = get_post_thumbnail_id( $post_id );
wp_delete_attachment($attachment_id, true);
}
}