File upload to asp.net core server using httpclient

I'm gonna answer my own question, very hard to find how to do file upload using httpclient from a user app, and there is one important detail that I found that may unstuck someone. 

here is my client code:

public static async Task<string> SendFile(HttpClient client, string Content, string comment)
        {
            MultipartFormDataContent multiContent = new MultipartFormDataContent();

            var payload = Encoding.UTF8.GetBytes(Content);
            multiContent.Add(new ByteArrayContent(payload), "files", comment); // name must be "files"
            var response = await client.PostAsync($"{BaseUrl}/api/myFile/Upload", multiContent).ConfigureAwait(false);
            return await response.Content.ReadAsStringAsync().ConfigureAwait(false);
        }

Here is the controller

public async Task<IActionResult> Upload(List<IFormFile> files)
        {
            if (!ModelState.IsValid)
                return BadRequest("Invalid Input");
            if (files == null || files.Count != 1 )
                return BadRequest("Invalid files");
            var file = files[0];
            if (file == null || file.Length == 0 || file.Length > 1 * 1024 * 1024)
            return BadRequest("Invalid file");

            string Content;
            using (MemoryStream ms = new MemoryStream())
            {
                await file.CopyToAsync(ms).ConfigureAwait(false);
                Content = Encoding.ASCII.GetString(ms.ToArray());
                 
            }
            // Content is now the uploaded text file 

            return Ok();
        }

the trick that was not clearly documented was that in 

multiContent.Add(new ByteArrayContent(payload), "files", comment); // name must be "files"

That field name "files" must be "files", not "file" as I originally had it. With the incorrect  name, I would get to the controller, but the List<IFormFile> files would have no files (length == 0). with not specifying a List just(IFormFile file), file would be null. Odd part about it is that in both these cases, ModelState.IsValid would be true even though no file is passed in.