request.txt

public T Post<T>(string path, IEnumerable<KeyValuePair<string, string>> parameters, FilePayload payload)
{
    using (var restClient = new RestClient())
    {
        using (var content = new MultipartFormDataContent())
        {
            content.Add(new FormUrlEncodedContent(parameters));

            var streamContent = new StreamContent(new MemoryStream(payload.ContentBytes));
            content.Add(streamContent, "attachment1", payload.FileName);

            var postMessage = restClient.CreateMessage(BuildUrl(path), HttpMethod.Post, _accessToken, content);
            var task = restClient.SendAsync(postMessage);

            return restClient.DeserializeResult<T>(task);
        }
    }
}

POST https://www.yammer.com/api/v1/messages.json HTTP/1.1
Authorization: Bearer REMOVED
User-Agent: YammerApiClient (www.brianlyttle.com)
Content-Type: multipart/form-data; boundary="f1899122-117b-436b-a610-412771d6efa8"
Host: www.yammer.com
Content-Length: 9606
Expect: 100-continue
Connection: Keep-Alive

--f1899122-117b-436b-a610-412771d6efa8
Content-Type: application/x-www-form-urlencoded
Content-Disposition: form-data

body=This+is+a+test+from+code.
--f1899122-117b-436b-a610-412771d6efa8
Content-Disposition: form-data; name=attachment1; filename=mike.jpg; filename*=utf-8''mike.jpg

�����JFIF�


�H�H�����"Exif��MM�*����

����
�
��������XICC_PROFILE�

��






///// THE FOLLOWING HTTP REQUEST USING KDAVIE'S OLD CODE WORKED FINE //////

POST https://www.yammer.com/api/v1/messages/ HTTP/1.1
Content-Type: multipart/form-data; boundary=XZQVNHVS[DMLVOXFWILMERKNZI
Authorization: Bearer REMOVED
Cache-Control: no-cache
Host: www.yammer.com
Content-Length: 395490
Expect: 100-continue
Connection: Keep-Alive

--XZQVNHVS[DMLVOXFWILMERKNZI
Content-Disposition: form-data; name="body"

This is a message
--XZQVNHVS[DMLVOXFWILMERKNZI
Content-Disposition: form-data; name="attachment1"; filename="mike.jpg"
Content-type: text/xml

�����JFIF�


�H�H����XICC_PROFILE�

��