can't write logs to file with file name

[hungbu]

var now = new DateTime.now();
    var formatter = new DateFormat('yyyy_MM_dd');
    String formattedDate = formatter.format(now);
    String logName = formattedDate + '_workspacelog';
    print(logName);
    FlutterLogs.logToFile(
      logFileName: logName,
        overwrite: true,
        //If set 'true' logger will append instead of overwriting
        logMessage:
        "This is a log message: ${DateTime.now().millisecondsSinceEpoch}",
        appendTimeStamp: true);

but file save as file name default :