Send files from flutter web to .net api

Question

I'm trying to send files to .net api, but api response me

{"mensaje":"startIndex cannot be larger than length of string. (Parameter 'startIndex')"}

  void _handleResult(Object? result) {
    setState(() {
      _bytesData =  Base64Decoder().convert(result.toString().split(",").last);
      _selectedFile = _bytesData;
    });
  }

Future<String?> makeRequest() async {
  var url = Uri.parse("http://*****._____.com.bo:88/serviciosDocumento/api/DocumentosAdicionar");
  var request = new http.MultipartRequest("POST", url)..fields['usuIdR'] = "8";

  print(_selectedFile);
  request.files.add(await http.MultipartFile.fromBytes('request', _selectedFile,
    // contentType: new MediaType('application', 'octet-stream'),
    filename: "file_up"));

  //send
  var response = await request.send();
  print(response.statusCode);

  // listen for response
  response.stream.transform(utf8.decoder).listen((value) {
    print(value);
  });

}

This error not appears when compile in mobile