无法解压gzipstream

从一网站上来载的xls，是用gzip压缩的，下面的代码不能解压，gzipstream.read()啥也没有。如果我直接将mem写入文件，然后用winrar解压是正确的。 bool ret = false;
            byte[] bs = Encoding.ASCII.GetBytes(url);
            Stream reqStream = null;
            Stream stream = null;
            StreamReader streamReader = null;
            HttpWebResponse webResponse = null;
            HttpWebRequest req = null;            try
            {
                req = (HttpWebRequest)HttpWebRequest.Create(url);
                req.Method = "POST";
                req.ContentType = "application/x-www-form-urlencoded";
                req.ContentLength = bs.Length;
                req.CookieContainer = SpeedCookie;
                req.Headers[HttpRequestHeader.AcceptEncoding] = "gzip, deflate";                reqStream = req.GetRequestStream();
                reqStream.Write(bs, 0, bs.Length);
                webResponse = (HttpWebResponse)req.GetResponse();
                stream = webResponse.GetResponseStream();//以流的形式读取
                MemoryStream mem = new MemoryStream();                //先写入内存，再解压
                Byte[] buf = new Byte[4096];
                int count = stream.Read(buf, 0, 4096);
                while (count > 0)
                {
                    mem.Write(buf, 0, count);
                    count = stream.Read(buf, 0, 4096);
                }mem中的数据是没有问题的，我直接将mem写入文件，然后用winrar打开是对的
                //GZIP解压先
                GZipStream zip = new GZipStream(mem, CompressionMode.Decompress);
                FileStream file = new FileStream(path, FileMode.Create);
                count = zip.Read(buf, 0, 4096);
                while (count > 0)
                {
                    file.Write(buf, 0, count);
                    count = zip.Read(buf, 0, 4096);
                }                file.Close();
                mem.Close();
            }
我用http的stream来构造gzipstream，也啥都没读出来，返回的头是这样的：
Content-Disposition: attachment;filename=summary_2013_04_13_11_06_52.xls.gz
Content-Length: 2346
Content-Type: application/x-gzip;charset=utf-8
Date: Sat, 13 Apr 2013 03:06:53 GMT
Server: Apache-Coyote/1.1