postfile.cs

public class PostFile
{
    public T Post<T>(string path, string text, List<Attachment> attachments)
    {
        using (var restClient = new RestClient())
        using (var content = new MultipartFormDataContent())
        {
            // Message part
            var messageContent = new StringContent(text);

            var messageDispositionValue = new ContentDispositionHeaderValue("form-data")
            {
                Name = "\"body\""
            };

            messageContent.Headers.ContentDisposition = messageDispositionValue;
            content.Add(messageContent);

            // File attachment part
            int index = 1;
            foreach (var attachment in attachments)
            {
                var byteContent = new ByteArrayContent(attachment.ContentBytes);
                byteContent.Headers.ContentType = MediaTypeHeaderValue.Parse("text/xml");

                var name = string.Format("attachment{0}", index);
                var imageDispositionValue = new ContentDispositionHeaderValue("form-data")
                {
                    Name = string.Format("\"{0}\"", name),
                    FileName = string.Format("\"{0}\"", attachment.FileName)
                };

                byteContent.Headers.ContentDisposition = imageDispositionValue;
                content.Add(byteContent);

                index++;
            }

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

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

public class RestClient : HttpClient
{
    public RestClient()
    {
        SetUserAgent("YammerApiClient (www.brianlyttle.com)");
    }

    /// <summary>
    /// Sets a custom user agent for all requests sent by the client.
    /// </summary>
    /// <param name="userAgent">String containing the user agent.</param>
    public void SetUserAgent(string userAgent) 
    {
        if (userAgent == null) 
            throw new ArgumentNullException("userAgent");

        DefaultRequestHeaders.Add("User-Agent", userAgent);
    }

    /// <summary>
    /// Returns an HTTP message without content.
    /// </summary>
    /// <param name="address">Complete URL for service</param>
    /// <param name="method">HTTP method</param>
    /// <param name="token">OAuth token</param>
    /// <returns>HttpRequestMessage</returns>
    public HttpRequestMessage CreateMessage(Uri address, HttpMethod method, string token)
    {
        if (token == null) throw new InvalidTokenException("token");

        var message = new HttpRequestMessage
        {
            RequestUri = address,
            Method = method
        };

        message.Headers.Add("Authorization", string.Format("Bearer {0}", token));

        return message;
    }

    /// <summary>
    /// Returns an HTTP message with content.
    /// </summary>
    /// <param name="address">Complete URL for service</param>
    /// <param name="method">HTTP method</param>
    /// <param name="token">OAuth token</param>
    /// <param name="content">Content</param>
    /// <returns>HttpRequestMessage</returns>
    public HttpRequestMessage CreateMessage(Uri address, HttpMethod method, string token, HttpContent content)
    {
        if (token == null) throw new InvalidTokenException("token");

        var message = new HttpRequestMessage
        {
            RequestUri = address,
            Method = method,
            Content = content
        };

        message.Headers.Add("Authorization", string.Format("Bearer {0}", token));

        return message;
    }

    public HttpRequestMessage CreateMessage(Uri address, HttpMethod method, string token, MultipartFormDataContent content)
    {
        if (token == null) throw new InvalidTokenException("token");

        var message = new HttpRequestMessage
        {
            RequestUri = address,
            Method = method,
            Content = content
        };

        message.Headers.Add("Authorization", string.Format("Bearer {0}", token));

        return message;
    }

    /// <summary>
    /// Deserializes a response from a service.
    /// </summary>
    /// <typeparam name="T">Type of expected JSON response</typeparam>
    /// <param name="task">Task holding the HttpResponseMessage</param>
    /// <returns>Type of expected JSON response</returns>
    public T ProcessResult<T>(Task<HttpResponseMessage> task)
    {
        if (task == null) throw new ArgumentNullException("task");

        // Throw exception on request failure
        task.Result.EnsureSuccessStatusCode();

        // Deserialize if we have a result
        return JsonConvert.DeserializeObject<T>(task.Result.Content.ReadAsStringAsync().Result);
    }

    /// <summary>
    /// Converts a list of parameters to a query string (sans URL encoding).
    /// </summary>
    /// <param name="parameters">IEnumerable of KeyValuePair</param>
    /// <returns>string</returns>
    public string FormatUrlParameters(IEnumerable<KeyValuePair<string, string>> parameters)
    {
        if (parameters == null) return "";

        var sb = new StringBuilder("?");
        foreach (var keyValuePair in parameters)
        {
            sb.AppendFormat("{0}={1}&", keyValuePair.Key, keyValuePair.Value);
        }

        return sb.ToString();
    }
}

public class Attachment
{
    public string FileName { get; set; }
    public byte[] ContentBytes { get; set; }

    public Attachment(string name, byte[] content)
    {
        FileName = name;
        ContentBytes = content;
    }
}