public function setExtension($sExtension = null)
    {
        if (null === $sExtension) {
            return $this;
        }

        if (!in_array($sExtension, $this->aValidExtensions)) {
            $message = sprintf(
                'The extension "%s" is not a valid one, extension image for Gravatar can be: %s',
                $sExtension,
                implode(', ', $this->aValidExtensions)
            );

            throw new InvalidImageExtensionException($message);
        }

        $this->sExtension = $sExtension;

        return $this;
    }