function drag_and_drop_file_validation() {
    ?>
    <script>
    jQuery(document).ready(function($) {
        $('button[type="submit"]', $('form.cart')).on("click", function(e) {
            var $file = $('input.wc-drag-n-drop-file');
            var $total_files = $('input[name^="' + $file.data('name') + '"]').length;
            $('.has-error-msg').remove();

            if ($total_files == 0) {
                $('.codedropz--results').after('<span class="has-error-msg">File upload is required.</span>');
                return false;
            }
        });
    });
    </script>
    <?php
}
add_action('wp_footer', 'drag_and_drop_file_validation');