I don’t have experience with REST APIs. I inherited a .NET 4.7.1 C# client/server and need to add (large) binary file transfer capability.

On the client side, this is how far I’ve gotten:

HttpClient client = new HttpClient();
...
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(?); // TODO: What should I enter here?

using (var content = new MultipartFormDataContent())
{
   var fileContent = new ByteArrayContent(bytes);
   fileContent.Headers.ContentType = MediaTypeHeaderValue.Parse("multipart/form-data");
   fileContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
   {
      FileName = "My.dat"
   };
   content.Add(fileContent);
   var response = await client.PostAsync(url, content);
   response.EnsureSuccessStatusCode();
   return response.Headers.Location;
}

Post fails with:

Status Code: 500, ReasonPhrase: 'could not find expected boundary'

I’m not married to the above implementation (found on the web). I can’t use RestSharp, though.

On the server side, how do I extract the bytes from the request parameter?

public async Task ReceiveFile(
         HttpListenerRequest request,
         ...)
{
   ...
   if (request.HasEntityBody)
   {
      // TODO: Save bytes to a file
   }
   ...
}

I am limited to a handler that gets an HttpListenerRequest parameter.