How can I upload files with the form element type file?

Question

I want to upload a file and I have to use form element type as 'file'. In submit function I am not able to get any proper data related to the file which I am uploading. SO Can anyone please help me to resolve this. Please find the below code:

public function buildForm(array $form, FormStateInterface $form_state) {
  $form['test_CERTIFICATE'] = [
    '#type' => 'file',
    '#title' => $this->t('Certificate'),
    '#description' => $this->t('Your Certificate (.pem file)').': '.\Drupal::state()->get('test_CERTIFICATE_NAME'),           
  ];
  
  return parent::buildForm($form, $form_state);
}

Submit function:

public function submitForm(array &$form, FormStateInterface $form_state) {
  parent::submitForm($form, $form_state);

  $validators = array('file_validate_extensions' => array('pem'));
  $files = file_save_upload('test_CERTIFICATE', $validators, 'public://certfiles', FILE_EXISTS_REPLACE);
  $file = File::load($files[0]);
  if ($file) {
    kint($files); exit;
    // here control is not coming
    $file->setPermanent();
    $file->save();
  }
}

Answer 1

I had the same problem with the file form element. Solved this by using managed_file and providing the upload location and validators in the form element:

$form['test_CERTIFICATE'] = [
  '#type' => 'managed_file',
  '#title' => $this->t('Certificate'),
  '#upload_location' => 'private://certfiles',
  '#upload_validators' => [
    'file_validate_extensions' => ['pem'],
  ],
];

Then in submit:

use Drupal\file\Entity\File;

$form_file = $form_state->getValue('test_CERTIFICATE', 0);
if (isset($form_file[0]) && !empty($form_file[0])) {
  $file = File::load($form_file[0]);
  $file->setPermanent();
  $file->save();
}

Answer 2

You can access files data uploaded through "file" field using the following code (from D8.5 core/modules/config/src/Form/ConfigImportForm.php)

$all_files = $this->getRequest()->files->get('files', []);
$file = $all_files['test_CERTIFICATE'];
$file_path = $file->getRealPath();