Problems getting instance of UploadedFile in Yii2

php file-upload yii2

PHP Snippet 1:

// View
<?= $form->field($model, 'arch')->fileInput(); ?>

// Controller
$model->arch = UploadedFile::getInstanceByName('arch');

PHP Snippet 2:

'options' => ['enctype' => 'multipart/form-data']

PHP Snippet 3:

$names = UploadedFile::getInstances($model, 'filename');