exemple custom managed_file in custom block

  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $form['header'] = array(
      '#type' => 'fieldset',
      '#title' => t('Header'),
    );
    $form['header']['subtitle'] = array(
      '#type' => 'textfield',
      '#tree' => TRUE,
      '#title' => t('Subtitle'),
      '#default_value' => $this->configuration['header']['subtitle'],
    );
    $form['header']['title'] = array(
      '#type' => 'text_format',
      '#tree' => TRUE,
      '#title' => t('Title'),
      '#default_value' => $this->configuration['header']['title']['value'],
      '#format' => $this->configuration['header']['title']['format'],
    );
    $form['header']['fid'] = array(
      '#title' => t('Background image'),
      '#type' => 'managed_file',
      '#default_value' => !empty($this->configuration['header']['fid']) ? $this->configuration['header']['fid'] : NULL,
      '#upload_location' => 'public://images/block',
    );

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    // Get old value.
    $element = $form['settings']['header']['fid'];
    $fid_old = isset($element['#default_value']) ? $element['#default_value'] : array();

    $fid = $form_state->getValue(array('header', 'fid'), array());

    if (!empty($fid) && $fid != $fid_old) {
      $fid = reset($fid);

      $file = file_load($fid);
      $file->status = FILE_STATUS_PERMANENT;
      $file->save();

      \Drupal::service('file.usage')->add($file, 'mycustom_block', 'block', 1);
    }
    if (!empty($fid_old) && $fid != $fid_old) {
      // Value has changed - delete old file.
      $fid_old = reset($fid_old);

      $file = file_load($fid_old);
      if ($file) {
        // Delete file usage and let garbage colector take care of actual file.
        \Drupal::service('file.usage')->delete($file, 'mycustom_block', 'block', 1);
      }
    }

    $this->configuration['header']['subtitle'] = $form_state->getValue(array('header', 'subtitle'), '');
    $this->configuration['header']['title'] = $form_state->getValue(array('header', 'title'), '');
    $this->configuration['header']['fid']   = $form_state->getValue(array('header', 'fid'), array());
  }