[ticket/17443] Fix HTTP 403 response to Guzzle client requests for some hosts

PHPBB-17443

phpBB/phpbb/file_downloader.php

@@ -51,6 +51,10 @@ class file_downloader
 		return new Client([
 			'base_uri' => $host,
 			'timeout'  => $timeout,
+			'headers' => [
+				'user-agent' => 'phpBB/' . PHPBB_VERSION,
+				'accept' => 'text/html,application/xhtml+xml,application/xml'
+			  ],
 		]);
 	}