void silc_sftp_extended(SilcSFTP sftp,
                            const char *request,
                            const unsigned char *data,
                            SilcUInt32 data_len,
                            SilcSFTPExtendedCallback callback,
                            void *context);

    Performs an extended operation indicated by the `request' with
    optional extended operation data indicated by the `data'.  The callback
    is called to return any data associated with the extended request.