How to attach a BitMap (or an image) to Slack message as attachment [C#]

Question

I am using IHttpClientFactory and doing a POST request with parameters to receive an image. Then I need to attach this image to the slack message. In a normal way, we have to publish our image to a public cloud then attach the image URL to the slack attachment BUT I cannot do so because the image is private. This is my current code.

using (HttpClient httpClient = _clientFactory.CreateClient())
                {
                    httpClient.DefaultRequestHeaders.Add("Accept", "image/*");
                    var parameters = new
                    {
                        names = "name x, name y",
                        title = "some title!",
                        msg = @"msg body",
                        time = "",
                        images = "",
                    };
                    var serializedParams = JsonConvert.SerializeObject(parameters);
                    HttpContent paramContent = new StringContent(serializedParams, Encoding.UTF8, "application/json");

                    using (HttpResponseMessage response = await httpClient.PostAsync($"{"the nedpoint", paramContent, new CancellationToken()))
                    {
                        if(response != null && response.IsSuccessStatusCode)
                        {
                            using(var stream = await response.Content.ReadAsStreamAsync())
                            {
                                using (var mStream = new MemoryStream())
                                {
                                    await stream.CopyToAsync(mStream);
                                    generatedImage = new Bitmap(System.Drawing.Image.FromStream(mStream));

                                }
                            }
                        }
                    }



                }

Now I want to attach the generatedImage to the following attachment

var model = new
            {
                id = Guid.NewGuid().ToString(),
                type = "message",
                text = "",
                attachments = new[]
                {
                    new {
                        color = "#36a64f",
                        title = messageTitle,
                        text = messageBody
                        }
                }
            };
            var client = _clientFactory.CreateClient();
            client.Timeout = new TimeSpan(0, 0, 30);
            var serializedContent = JsonConvert.SerializeObject(model);
            client.DefaultRequestHeaders.Add("Accept", "application/json");
            HttpContent content = new StringContent(serializedContent, Encoding.UTF8, "application/json");

            await client.PostAsync(channel, content, new CancellationToken());