upload a file in a drupal 8 contributed module - drupal

Question

I am trying to create my first drupal 8 module. in this module I have to create a new form and provide user a file uploading capability in this form. here is my form controller:
class Make2d extends FormBase {
/**
* {#inheritdoc}
*/
public function getFormId() {
return 'make2d_form';
}
/**
* {#inheritdoc}
*/
public function buildForm(array $form, FormStateInterface $form_state) {
if (\Drupal::currentUser()->isAnonymous()) {
return new RedirectResponse(\Drupal::url('user.page'));
}
$form['sheet_size'] = array(
'#type' => 'radios',
'#title' => t('Sheet Size'),
'#options' => array(t('10 X 10(2.99$)'), t('17 X 17(4.99$)'), t('28 X 28(5.99$)')),
);
$form['uploaded_file'] = array(
'#type' => 'file',
'#title' => t('Upload your file'),
'#required' => true
);
$form['actions']['#type'] = 'actions';
$form['actions']['submit'] = array(
'#type' => 'submit',
'#value' => $this->t('Save to Cart'),
'#button_type' => 'primary',
);
return $form;
}
/**
* {#inheritdoc}
*/
public function validateForm(array &$form, FormStateInterface $form_state) {
echo '<pre>';
print_r($form_state->getvalues());
echo '</pre>';
}
/**
* {#inheritdoc}
*/
public function submitForm(array &$form, FormStateInterface $form_state) {
print_r($form_state['values']);
}
}
this is the result when I go to my form's page:
image of my form
then I choose a file from my computer and submit the form. but when I print_r my $form_state->getvalues() array the result is sth like this:
Array
(
[sheet_size] => 0
[uploaded_file] =>
[submit] => Drupal\Core\StringTranslation\TranslatableMarkup Object
...
you can see that [uploaded_file] is empty. and there is an error on top of the form about uploading a file. what is wrong with the form controller and file uploading.
thanks.

Answer 1

at last! I find it. we should use '#type' = 'managed_file' so that we let drupal to manage uploaded file. in the case we use '#type' = 'file' we must transfer file ourselves by file_save_upload().