When doing a migration of Help Center articles, I ran into some trouble using the API to upload attachments to articles. This is a sample of how I was able to achieve this task.

private void UploadHelpAttachment(string fileName, string filePath)

    {

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://{subdomain}.zendesk.com/api/v2/help_center/articles/{articleid}/attachments.json?inline=true");

        request.Method = "POST";

        request.Accept = "application/json, application/xml, text/json, text/x-json, text/javascript, text/xml";

        request.Timeout = 100000000;

        request.Headers.Add("Authorization", "Basic " + token);

        using (var dataStream = request.GetRequestStream())

        {

            var data = GetFromData(fileName, filePath, request);

            dataStream.Write(data, 0, data.Length);

        }

        var res = request.GetResponse();

        var response = res as HttpWebResponse;

        string responseFromServer = string.Empty;

        using (var responseStream = response.GetResponseStream())

        {

            using (var reader = new StreamReader(responseStream))

            {

                responseFromServer = reader.ReadToEnd();

            }

        }

        Response.Clear();

        Response.ContentType = "application/json";

        Response.Write(responseFromServer);

        Response.End();

    }

    private byte[] GetFromData(string fileName, string filePath, HttpWebRequest request)

    {

        var attachment = System.IO.File.ReadAllBytes(filePath);

        string boundaryString = new Guid.NewGuid().ToString();

        MemoryStream postDataStream = new MemoryStream();

        StreamWriter postDataWriter = new StreamWriter(postDataStream);

        request.ContentType = "multipart/form-data; boundary=" + boundaryString;

        postDataWriter.Write("\r\n--" + boundaryString + "\r\n");

        postDataWriter.Flush();

        postDataWriter.Write("Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n", "file", fileName, MimeMapping.GetMimeMapping(fileName));

        postDataWriter.Flush();

        postDataStream.Write(attachment, 0, attachment.Length);

        postDataWriter.Write("\r\n--" + boundaryString + "--\r\n");

        postDataWriter.Flush();

        return postDataStream.ToArray();

    }