.net4.0 调用API（form-data）上传文件及传参

/// <summary>
        /// 
        /// </summary>
        /// <param name="url">API URL</param>
        /// <param name="token">API token</param>
        /// <param name="parameters">API参数(除文件类型外)</param>
        /// <param name="fileContent">文件参数(二进制文件流)</param>
        /// <param name="fileName">文件名</param>
        /// <returns></returns>
        public string Post_formdata(string url, string token, Dictionary<string, string> parameters, byte[] fileContent, string fileName)
        {
            string result = "";
            string strBoundary = "--" + DateTime.Now.Ticks.To