createFileItem example

throw new \InvalidArgumentException('Can only add media types which use a file field as a source field.');
    }
    return $media_type;
  }

  /** * {@inheritdoc} */
  protected function buildInputElement(array $form, FormStateInterface $form_state) {
    // Create a file item to get the upload validators.     $media_type = $this->getMediaType($form_state);
    $item = $this->createFileItem($media_type);

    /** @var \Drupal\media_library\MediaLibraryState $state */
    $state = $this->getMediaLibraryState($form_state);
    if (!$state->hasSlotsAvailable()) {
      return $form;
    }

    $slots = $state->getAvailableSlots();

    // Add a container to group the input elements for styling purposes.     $form['container'] = [