managed_file file extension validation issues (different issue)

File extension validation fails if you try to upload a file like 'fred.php' or 'jenny'. This seems to be because these types of files are automatically renamed for security purposes. This rename, stops the file extension validation from being called it seems.

public function buildForm(array $form, FormStateInterface $form_state) {
    $form['document_scan'] = array
    (
      '#type' => 'managed_file',
      '#title' => t('Document (Scanned Copy)'),
      '#description' => t('Upload a file. Allowed extensions: pdf, jpg, jpeg, png, gif'),
      '#required' => TRUE,
      '#upload_validators'  => array
      (
        'file_validate_extensions' => array('pdf jpg jpeg png gif'),
        'file_validate_size' => array(25600000),
      ),
      '#upload_location' => 'public://myfile/'
    );