request.AddParameter("text/plain", body, ParameterType.RequestBody);

public static void UploadFile(string bearer)
    {
        var client = new RestClient("...");
        var request = new RestRequest(Method.POST);
        request.AddHeader("Authorization", "Bearer " + bearer);
        request.AddHeader("cache-control", "no-cache");

        var imageLenngth = new FileInfo(@"...").Length;

        request.AddHeader("Content-Type", "image/jpeg");
        request.AddHeader("Content-Length", imageLenngth.ToString());

        byte[] body = File.ReadAllBytes(@"...");

        request.AddParameter("text/plain", body, ParameterType.RequestBody);

        IRestResponse response = client.Execute(request);
    }