function setContentType( $typeAlias )
    {
        // expand short-hand notation for "php", json" etc
        if ( isset( self::$KnownContentTypes[$typeAlias] ) )
        {
            $this->ContentType = self::$KnownContentTypes[$typeAlias];
            return true;
        }
        // accept as well other mimetypes, as long as we later know how to encode them
        $decodableType = $this->mimeTypeToEncodeType( $typeAlias );
        if ( isset( self::$KnownContentTypes[$decodableType] ) )
        {
            $this->ContentType = $typeAlias;
            return true;
        }
        return false;
    }