$username = 'SITE USERNAME';
$password = 'SITE PASSWORD';
$url = 'SITE URL';
$response = Drupal::httpClient()->get($url . "/entity/file/FILE ID?_format=json", [
  'auth' => [$username, $password],
  'headers' => [
    'Content-Type' => 'application/json',
  ],
]);
$json_string = (string) $response->getBody();