xamarin/xamarin-android

public async Task UploadImageAsync(Stream image, string fileName)
{
string url = "http://localhost/api/ImagesUpload";
HttpContent fileStreamContent = new StreamContent(image);
fileStreamContent.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("form-data") { Name = "file", FileName = fileName };
fileStreamContent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/octet-stream");
using (var client = new HttpClient())
using (var formData = new MultipartFormDataContent())
{
formData.Add(fileStreamContent);
var response = await client.PostAsync(url, formData);
var HtmlResult = await response.Content.ReadAsStringAsync();
var result = JsonConvert.DeserializeObject<string>(HtmlResult);
}
}

Hope you got over that issue.

It would have been informative to include any exceptions to clarify what then its not uploaded implies.

Make sure the stream image is not closed and image.Position == 0.
If the stream was written to, it´s most likely that the position is pointing to the end. When you attempt to upload it would already be at the end of the stream.

NOTE: you might have gotten answers on StackOverflow as well