gistfile1.cs

public async Task<HttpResponseMessage> SendAsync(IDictionary<string, object> parameter, AsyncOAuth.AccessToken accessToken, string footer)
{
	var client = new HttpClient();
	var boundary = DateTime.Now.ToString("MMddHHmmss") + new Random().Next().ToString();
	var content = new MultipartFormDataContent(boundary);
	foreach (var item in parameter)
	{
		if (item.Value is string)
		{
			content.Add(new StringContent(item.Value as string), "\"" + this.MessageKeyName + "\"");
		}
		else if (item.Value is Stream)
		{
			var sendData = await Task.Factory.StartNew<byte[]>(() =>
			{
				var ms = item.Value as MemoryStream;
				// ストリームをバイト配列にする
				ms.Seek(0, SeekOrigin.Begin);

				var send = new List<byte>();
				while (true)
				{
					var rb = new byte[1024];
					var readBytes = ms.Read(rb, 0, rb.Length);
					if (readBytes <= 0)
						break;

					send.AddRange(rb);
				}

				return send.ToArray();
			});

			content.Add(new ByteArrayContent(sendData), "\"" + this.MediaKeyName + "\"", string.Format("\"upload_{0}.jpg\"", boundary));
		}
	}

	var result = await OAuthUtility.CreateOAuthClient(this.ConsumerKey, this.ConsumerSecret, accessToken)
		.PostAsync(this.ServiceUri, content);

	var response = await result.Content.ReadAsStringAsync();
	Console.WriteLine(response);

	return result;
}

gistfile2.txt

response:
{"errors":[{"code":189,"message":"Error creating status."}]}