This is the right hook to do this:
add_action('before_delete_post', 'delete_post_attachments');
function delete_post_attachments($post_id){
global $wpdb;
global $post_type;
if (!$post_type == 'bkroadkill')
return;
$args = array(
'post_type' => 'attachment',
'post_status' => 'any',
'posts_per_page' => -1,
'post_parent' => $post_id
);
$attachments = new WP_Query($args);
$attachment_ids = array();
if($attachments->have_posts()) : while($attachments->have_posts()) : $attachments->the_post();
$attachment_ids[] = get_the_id();
endwhile;
endif;
wp_reset_postdata();
if(!empty($attachment_ids)) :
$delete_attachments_query = $wpdb->prepare('DELETE FROM %1$s WHERE %1$s.ID IN (%2$s)', $wpdb->posts, join(',', $attachment_ids));
$wpdb->query($delete_attachments_query);
endif;
}