typedef void (*SilcSFTPExtendedCallback)(SilcSFTP sftp,
                                             SilcSFTPStatus status,
                                             const unsigned char *data,
                                             SilcUInt32 data_len,
                                             void *context);

    Extended request callback is called when client sends extended
    request to the server. The `data' is arbitrary data returned by the
    server and its encoding is the extended request specific.