hook_file_download() was removed and party replaced with hook_file_unmanaged_download_headers()

hook_file_download() was removed. Response headers for managed files are now handled automatically by the entity object. Headers for unmanaged files are no controlled via hook_file_unmanaged_download_headers(), which was introduced in Drupal 8. New hook works very similar to the old one, except it only affects unmanaged files.