Thumbnails not generated when using private filesystem

This is looking good.

Thumbnails are now being generated as expected.

Hello,
Field "YouTube thumbnail directory" has the public files path prefix,

and problem could appear if:
* Site has the private files directory within the public one
* User has selected private files directory to store the thumbnails.

Only in this case you will have this issue.
And if module user has selected private files directory, he has a reason to hide this files from the users which don't have this permission.

Works as designed, nothing to fix there.

In this case we have issues:
1) In the hook we don't check if user has an access to the field content (to the entity, at least). We just give all the users access to the thumbnails in any case.
2) We don't allow users to define private:// file directory in case if it located outside the public one.

We had the same issue on a Drupal 8 website but got it to work with a simple download hook:

/**
 * Implements hook_file_download().
 *
 * @throws PluginNotFoundException
 * @throws InvalidPluginDefinitionException
 */
function xxx_tweaks_file_download($uri) {
  if (strpos(file_uri_target($uri), Drupal::config('youtube.settings')->get('youtube_thumb_dir')) === 0) {
    /** @var \Drupal\Core\Entity\EntityTypeManager $entityManager */
    $entityManager = Drupal::entityTypeManager();
    $files = $entityManager->getStorage('file')->loadByProperties(['uri' => $uri]);
    return file_get_content_headers(current($files));
  }
}