Upload multiple files when placing a WooCommerce order

I am using a code that adds a file upload field on the checkout page. How to add file upload to WooCommerce checkout? Many thanks to @LoicTheAztec for this code.

I have created the files checkout_uploads.php, checkout_upload.js and added the code to functions.php a child theme.

add_action( 'woocommerce_after_order_notes', 'add_custom_checkout_field' );
function add_custom_checkout_field($checkout) {

    echo '<div class="woocommerce-additional-fields__field-wrapper">';

    woocommerce_form_field('certificate', array(
        'type'      => 'file',
        'class'     => array('form-row-wide'),
        'label'     => __('File', 'woocommerce'),
        'required'  => false,
        'max_size'  => '2048',
        'accept'    => '.pdf,.doc,.docx,.rtf,.txt',
    ), '');

    echo '</div>';
}

// Save the uploaded file URL and name
add_action( 'woocommerce_checkout_create_order', 'save_checkout_uploaded_file', 10, 2 );
function save_checkout_uploaded_file( $order, $data ){
    if( $checkout_upload = WC()->session->get('checkout_upload') ) {
        $order->update_meta_data( '_checkout_upload', $checkout_upload ); 
    }
    WC()->session->__unset('checkout_upload');
}

// Display the uploaded file in admin orders
add_action('woocommerce_admin_order_data_after_billing_address', 'display_uploaded_file_in_admin_orders');
function display_uploaded_file_in_admin_orders( $order ){
    if( $checkout_upload = $order->get_meta( '_checkout_upload' ) ) {
        printf( '<p>%s <br><a href="%s">%s</a></p>', 
            __("File Uploaded:", 'woocommerce'), 
            $checkout_upload['file_url'], 
            $checkout_upload['file_name'] 
        );
    }
}

// Display the uploaded file in thankyou page
add_action('woocommerce_order_details_after_order_table', 'display_uploaded_file_in_thankyou');
function display_uploaded_file_in_thankyou ( $order ){
    if( $checkout_upload = $order->get_meta( '_checkout_upload' ) ) {
        printf( '<p>%s <br><a href="%s">%s</a></p>', 
            __("File Uploaded:", 'woocommerce'), 
            $checkout_upload['file_url'], 
            $checkout_upload['file_name'] 
        );
    }
}

// Display the uploaded file in emails
add_action('woocommerce_email_customer_details', 'display_uploaded_file_in_email');
function display_uploaded_file_in_email ( $order ){
    if( $checkout_upload = $order->get_meta( '_checkout_upload' ) ) {
        printf( '<p>%s <a href="%s">%s</a></p>', 
            __("File Uploaded:", 'woocommerce'), 
            $checkout_upload['file_url'], 
            $checkout_upload['file_name'] 
        );
    }
}

Only one file can be uploaded at the moment. How can I upload multiple files at the same time? I will be glad of your help!