In case you're looking for how to convert the file to a byte array for the postman request:

byte[] bytes = System.IO.File.ReadAllBytes(@"C:\temp\myFile.txt");
string bytesStr = string.Join(",", bytes);

This will result into a long string that looks like this:

"49,48,58,50,52,58,50,54,..."

And then use it in the postman request like so:

{
    "FileBytes":[49,48,58,50,52,58,50,54],
    "DeviceId": 12345
}

Well it's an invalid JSON object. StreamData has to be either a string or an object with a key. Try

{
"From":"donotreply@xxx.com",  
"To":["abc@xxx.com"],
"Cc":["abc@xxx.com"],  
"Bcc":["abc@xxx.com"],
"Subject":"Export",  
"TemplateName":"Agent",
"EmailType":"Agent",  
"Attachments":[{
    "Name":"wifi.txt",
    "Type":"txt",
    "StreamData": "NzI5OTk4NzgxNg0KDQoxIG0tNjczDQoNCjJtLTExMjINCjRtLTE1NzENCjgtMjE4Mw0KDQoxMDYxDQoNCjUwMCBtb2RlbSByZWZ1bmRhYmxl"
}]

}

Firstly, asp.net core does not support post multiple parameters from body, you need put these parameters into a model then you could post them from body.

Then, what you did should be post from query. But it seems does not support byte[] from query.

1. If you use JSON.NET, you could post clientId,dtName and append from query and post dtValues from body like below(Be sure your controller is declared with [ApiController]):

2. If you use System.Text.Json,with Sytem.Text.Json a byte array (byte[]) will be serialized as base64 string. They stated that they won't add support for byte[] to be serialized as number array in a github issue.

Custom JsonConverter:

public class ByteArrayConverter : JsonConverter<byte[]>
{
    public override byte[] Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        short[] sByteArray = JsonSerializer.Deserialize<short[]>(ref reader);
        byte[] value = new byte[sByteArray.Length];
        for (int i = 0; i < sByteArray.Length; i++)
        {
            value[i] = (byte)sByteArray[i];
        }

        return value;
    }

    public override void Write(Utf8JsonWriter writer, byte[] value, JsonSerializerOptions options)
    {
        writer.WriteStartArray();

        foreach (var val in value)
        {
            writer.WriteNumberValue(val);
        }

        writer.WriteEndArray();
    }
}

Register it:

services.AddControllers().AddJsonOptions(options =>
{
     options.JsonSerializerOptions.Converters.Add(new ByteArrayConverter());
}); 

Test in Postman:

BTW, not sure what is your scenario, if you post the byte array is something like file(e.g: image or .doc or .xlx file), you could use IFormFile and add [FromForm] before the parameter. Both JSON.NET or System.Text.Json supports receive IFormFile, no need custom JsonConverter here:

[Route("Upload")]
public IActionResult Upload(int clientId, string dtName, [FromForm]IFormFile dtValues, bool append)
{
        return Ok();
}

Post the data like below:

If you do not want to separately post them from query and body, you could put them into a model and post them from body.

Model:

public class TestModel
{
    public int clientId { get; set; }
    public string dtName { get; set; }
    public byte[] dtValues { get; set; }
    public bool append { get; set; }
}

Post in Postman:

Note: if you use System.Text.Json, you still need to custom JsonConverter like above option 2 I shared.