<?php

use \yii\helpers\Html;
use yii\widgets\ActiveForm;

?>

<div>
    <?php $form = ActiveForm::begin(['options' => ['id' => 'upload-file-form']]); ?>

    <?= $form->field($dynamic_model, $file_field_name)->fileInput()->label($file_field_label) ?>

    <div class="center-elements">
        <?= Html::submitButton('Save', ['class' => 'btn btn-success btn-expanded']) ?>
    </div>

    <?php ActiveForm::end() ?>
</div>