Updated Theme Code for MB File Advanced within a MB Group

<?php
$group = rwmb_meta( 'documents_step_1' );

// Field der:
$file_id = $group[ 'der' ] ?? null;
?>
<?php if (!empty($file_id)): ?>
<h3>Uploaded file</h3>
<?php $file = RWMB_File_Field::file_info( $file_id ); ?>
<li><a href="<?= $file['url']; ?>"><?= $file['name']; ?></a></li>
<?php endif; ?>