SolarWinds API - Attachment is uploading but not associating to parent

I'm trying to upload an attachment to an existing incident using the API. I can see I'm receiving a 201 HTTP status; however, when I look at the incident on SolarWinds no attachment is showing. The attachment exists though, and I'm able to navigate to it but no association/relationship.

            HttpClient client = new HttpClient();
            HttpRequestMessage message = new HttpRequestMessage();

            FileStream f = File.Open(attachment.AttachmentContent.Path, FileMode.Open);
            MultipartFormDataContent form = new MultipartFormDataContent();
            form.Add(new StringContent(attachment.AttachmentContent.Type), "file[attachable_type]");
            form.Add(new StringContent($"file[attachable_id]={Convert.ToInt32(attachment.AttachmentContent.Number)}"));
            form.Add(new StreamContent(f), "file[attachment]", Path.GetFileName(attachment.AttachmentContent.Path));

            message.RequestUri = new Uri(url);
            message.Content = form;
            message.Method = HttpMethod.Post;

            client.DefaultRequestHeaders.Add("X-Samanage-Authorization", "Bearer " + AuthenticationToken);
            HttpResponseMessage response = client.SendAsync(message).Result;
            LastResponse = response;

            if (!response.IsSuccessStatusCode) throw new ApplicationException("Didn't receive OK 200");
            return response.Content.ReadAsStringAsync().Result;

I'm baffled now and out of ideas. I hope someone is able to assist.

Note: I've put this in the wrong forum previously, not a repeat I'll be deleting the other one shortly.