protected function lowerFileMeta(array &$meta)
    {
        $metaHeaders = $this->getMetaHeaders();

        // get unregistered meta
        $unregisteredMeta = array();
        foreach ($meta as $key => $value) {
            if (!in_array($key, $metaHeaders)) {
                $unregisteredMeta[$key] = &$meta[$key];
            }
        }

        // Pico 1.0 lowered unregistered meta unsolicited...
        if ($unregisteredMeta) {
            $metaHeadersLowered = array_change_key_case($metaHeaders, CASE_LOWER);
            foreach ($unregisteredMeta as $key => $value) {
                $keyLowered = strtolower($key);
                if (isset($metaHeadersLowered[$keyLowered])) {
                    $registeredKey = $metaHeadersLowered[$keyLowered];
                    if ($meta[$registeredKey] === '') {
                        $meta[$registeredKey] = &$unregisteredMeta[$key];
                    }
                } else if (!isset($meta[$keyLowered]) || ($meta[$keyLowered] === '')) {
                    $meta[$keyLowered] = &$unregisteredMeta[$key];
                }
            }
        }
    }