SimpleMultipartFormRequestFileUpload.cs

Upload();

// https://docs.microsoft.com/en-us/aspnet/core/fundamentals/http-requests
// https://www.aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/
void Upload()
{
    Console.WriteLine("init - upload file");
    using (var client = new HttpClient())
    {
        client.DefaultRequestHeaders.TryAddWithoutValidation("User-Agent", "API Service");
        client.DefaultRequestHeaders.TryAddWithoutValidation("accept", "*/*");

        using (var content = new MultipartFormDataContent())
        {
            var path  = @"you_file_path";

            string fileName = Path.GetFileName(path);

            Console.WriteLine($"upload file name {fileName}");

            FileStream fs = File.OpenRead(path);

            var streamContent = new StreamContent(fs);              

            content.Add(streamContent, "file", Path.GetFileName(path));

            Console.WriteLine($"Post file {fileName} Async");          

            var url = "service_upload_url";
            
            Task<HttpResponseMessage> message = client.PostAsync(url, content);

            Console.WriteLine($" PostAsync");
            var input = message.Result.Content.ReadAsStringAsync();
            Console.WriteLine($"Result: {input.Result}");
            Console.Read();
        }
    }
}