MultipartFormDataContent

            StorageFile file;
            var content = new MultipartFormDataContent();
            content.Add(new StreamContent(await file.OpenStreamForReadAsync()), "name", "fileName");

            var client = new HttpClient();
            var request = new HttpRequestMessage(HttpMethod.Post, "url");
            request.Content = content;

            var response = await client.SendAsync(request);
            if (response.IsSuccessStatusCode)
            {

            }