在基于Http协议的Post请求中，Content-type为application/x-www-form-urlencoded的传输只能传送非文件的数据。

如果想用Http的Post方法来上传数据及文件，需要实现Content-type为multipart/form-data类型的协议程序。

下面是参考了StackOverflow网站上的一个例子，实现了关于客户端上传文件的功能类，代码如下：

using System;
using System.Net;
using System.Text;
using System.Collections.Generic;
using System.IO;

namespace ZDWorks.ZDClock.Cloud
{
    /// <summary>
    /// 文件类型数据的内容参数
    /// </summary>
    public class FileParameter
    {
        // 文件内容
        public byte[] File { get; set; }
        // 文件名
        public string FileName { get; set; }
        // 文件内容类型
        public string ContentType { get; set; }

        public FileParameter(byte[] file) : this(file, null) { }

        public FileParameter(byte[] file, string filename) : this(file, filename, null) { }

        public FileParameter(byte[] file, string filename, string contentType)
        {
            File = file;
            FileName = filename;
            ContentType = contentType;
        }
    }

    /// <summary>
    /// 数据与文件http请求
    /// </summary>
    public class HttpMultipartFormRequest
    {
        #region Data Members

        private readonly Encoding DefaultEncoding = Encoding.UTF8;
        private ResponseCallback m_Callback;
        private byte[] m_FormData;

        #endregion

        #region Constructor

        public HttpMultipartFormRequest()
        { 
        }

        #endregion

        #region Delegate

        public delegate void ResponseCallback(string msg);

        #endregion

        public void AsyncHttpRequest(string postUri, Dictionary<string, object> postParameters, ResponseCallback callback)
        {
            // 随机序列，用作防止服务器无法识别数据的起始位置
            string formDataBoundary = String.Format("----------{0:N}", Guid.NewGuid());
            // 设置contentType
            string contentType = "multipart/form-data; boundary=" + formDataBoundary;
            // 将数据转换为byte[]格式
            m_FormData = GetMultipartFormData(postParameters, formDataBoundary);
            // 回调函数
            m_Callback = callback;

            // 创建http对象
            HttpWebRequest request = HttpWebRequest.CreateHttp(new Uri(postUri));
            // 设为post请求
            request.Method = "POST";
            request.ContentType = contentType;
            // 请求写入数据流
            request.BeginGetRequestStream(GetRequestStreamCallback, request);
        }

        private void GetRequestStreamCallback(IAsyncResult ar)
        {
            HttpWebRequest request = ar.AsyncState as HttpWebRequest;
            using (var postStream = request.EndGetRequestStream(ar))
            {
                postStream.Write(m_FormData, 0, m_FormData.Length);
                postStream.Close();
            }
            request.BeginGetResponse(GetResponseCallback, request);
        }

        private void GetResponseCallback(IAsyncResult ar)
        {
            // 处理Post请求返回的消息
            try
            {
                HttpWebRequest request = ar.AsyncState as HttpWebRequest;
                HttpWebResponse response = request.EndGetResponse(ar) as HttpWebResponse;
                using (var stream = response.GetResponseStream())
                {
                    StreamReader reader = new StreamReader(stream);
                    string msg = reader.ReadToEnd();

                    if (m_