Removed attachment for custom post type after plugin deleted.

uninstall.php

function prefix_uninstall_attachments() {
    global $wpdb;

    $query = sprintf("
					SELECT ID
					FROM `%s`
					WHERE post_type = '%s'
						AND post_status NOT IN ( 'auto-draft', 'inherit' )
				", $wpdb->posts, {{custom_post_type}} );

    $post_ids = $wpdb->get_col($query);

    if (!$post_ids)
        return;

    $attachments = new WP_Query(array(
        'post_type'      => 'attachment',
        'posts_per_page' => -1,
        'post_status'    => 'any',
        'post_parent__in' => $post_ids,
        'no_found_rows'  => true,
        'fields'         => 'ids'
    ));
    $attachments = $attachments->posts;

    if ($attachments) {
        foreach ($attachments as $attachment) {
            wp_delete_attachment( $attachment );
        }
    }
}
prefix_uninstall_attachments();