private function fetchDomain($extension)
    {
        if (!array_key_exists($extension = strtolower($extension), $this->mediaDomainsMap)) {
            throw new UnsupportedFileException(sprintf(
                'Extension ".%s" isnt supported by Media component, only ["%s"] are.',
                $extension,
                implode('", "', array_keys($this->mediaDomainsMap))
            ));
        }

        return $this->mediaDomainsMap[$extension];
    }