Error while reading body of request message through JSON

I need to read content of message from the request body in WCF REST service like -

SERVICE SIDE CODE

string request = Encoding.UTF8.GetString(OperationContext.Current.RequestContext.RequestMessage.GetBody<byte[]>());

But I am getting an error on the service side, no matter what I try:

Expecting element 'base64Binary' from namespace 'http://schemas.microsoft.com/2003/10/Serialization/'.. Encountered 'Element' with name 'Human', namespace 'http://numans.hr-xml.org/2007-04-15'.

and the service contract is defined as:

 //[OperationContract(Name = "LoadMessages", IsOneWay = true)]
    [WebInvoke(Method = "POST",
        UriTemplate = "/LoadMessages",
        ResponseFormat = WebMessageFormat.Json,
        BodyStyle = WebMessageBodyStyle.Bare)]
    [Description("Inbound Message")]
    void LoadMessages();

and the implementation is as:

    public void LoadMessages()
    {
        string content = string.Empty;
        //var request = OperationContext.Current.RequestContext.RequestMessage.GetBody<FileState>();
        string request = Encoding.UTF8.GetString(OperationContext.Current.RequestContext.RequestMessage.GetBody<byte[]>());
 }

CLIENT SIDE CODE

Content that I'm sending is:

string jsonData = "{ \"categoryid\":\"" + file.CategoryId + "\",\"fileId\":\"" + file.FileId + "\" }";

I tried many options to send data from the client like:

var buffer = System.Text.Encoding.UTF8.GetBytes(jsonData);
var content = new ByteArrayContent(buffer);
content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

and also tried this:

var content = new StringContent(jsonData, Encoding.UTF8, "application/json");

Posting request:

 HttpResponseMessage executionResult = httpClient.PostAsync($"{url}/LoadMessages", content).Result;

I also tried serializing/de-serializing at client/server end, but that also is not working.

Can someone please suggest code samples what I can try that might work? Or point out what am I doing wrong.

A few more examples of what I tried with the JSON data :

 var jsonData = JsonConvert.SerializeObject(data, Formatting.Indented); 
 var details = JObject.Parse(data);

Pasting my client side function for clarity:

  HttpClient httpClient = new HttpClient(new HttpClientHandler());
  HttpStatusCode statusCode = HttpStatusCode.OK;
  string auditMessage = string.Empty;
  using (httpClient)
  {
     var url = ConfigurationManager.AppSettings["APIURL"];
     try
     {
        string jsonData = "{ \"categoryid\":\"" + file.CategoryId + "\",\"fileId\":\"" + file.FileId + "\" }";
                    
         //var jsonData = JsonConvert.SerializeObject(data, Formatting.Indented);
         //var details = JObject.Parse(data);

         //var content = new StringContent(jsonData, Encoding.UTF8, "application/json");
        var buffer = System.Text.Encoding.UTF8.GetBytes(jsonData);
        var content = new ByteArrayContent(buffer);
                    content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

        HttpResponseMessage executionResult = httpClient.PostAsync($"{url}/LoadMessages", content).Result;
        statusCode = executionResult.StatusCode;
        if (statusCode == HttpStatusCode.Accepted)
        {
          file.Status = "Success";
        }
      }
      catch (Exception ex)
      {
      }
    }

Here is my demo:

This is the interface document of the service:

This is the request:

class Program
{
    
    static void Main(string[] args)
    {
        
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://localhost:8012/ServiceModelSamples/service/user");
        request.Method = "POST";
        request.ContentType = "application/json;charset=UTF-16";
        string Json = "{\"Email\":\"123\",\"Name\":\"sdd\",\"Password\":\"sad\"}";
        request.ContentLength = Encoding.UTF8.GetByteCount(Json);
        Stream myRequestStream = request.GetRequestStream();
        StreamWriter myStreamWriter = new StreamWriter(myRequestStream, Encoding.GetEncoding("gb2312"));
        myStreamWriter.Write(Json);
        myStreamWriter.Close();

        HttpWebResponse response = (HttpWebResponse)request.GetResponse();

        Stream myResponseStream = response.GetResponseStream();
        //myResponseStream.ResponseSoapContext
        StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.GetEncoding("utf-8"));
        string retString = myStreamReader.ReadToEnd();
        myStreamReader.Close();
        myResponseStream.Close();
        Console.WriteLine(retString);
        Console.ReadKey();

    }
}

Feel free to let me know if the problem persists.

UPDATE

Define the Test class:

[DataContract]
    public class Test { 
    [DataMember]
    public string categoryid { get; set; }
    [DataMember]
    public string fileId { get; set; }
    }

the implementation is as:

public void LoadMessages(Test test)
    {
  Test dataObject = OperationContext.Current.RequestContext.RequestMessage.GetBody<Test>(new DataContractJsonSerializer(typeof(Test)));
           Console.WriteLine(dataObject.categoryid);
 }