download/file.php sends Content-Length header even when issuing 304 Not Modified

phpbb 3.0.12

download/file.php returns Content-Length header (with non-zero value) even if after it it returns 304 Not Modified. Which resulting to connection stuck until keepalive timeouts when server is nginx+php-fpm with keepalive_timeout > 0

There should not be Content-Length header at all if 304 is returned.