add_filter(
  'woocommerce_email_attachments',
  function( $attachments, $status, WC_Order $order ) {
    if ( ! is_object( $order ) || ! isset( $status ) ) {
      return $attachments;
    }

    if ( empty( $order ) ) {
      return $attachments;
    }

    if ( ! $order->has_downloadable_item() ) {
      return $attachments;
    }

    $allowed_statuses = array( 'customer_invoice', 'customer_completed_order' );

    if ( isset( $status ) && in_array( $status, $allowed_statuses ) ) {
      /**
       * @var WC_Order_Item_Product $item
       */
      foreach ( $order->get_items() as $item_id => $item ) {
        foreach ( $item->get_item_downloads() as $download ) {
          $attachments[] = str_replace( content_url(), WP_CONTENT_DIR, $download['file'] );
        }
      }
    }
    return $attachments;
  },
  10,
  3
);