模拟图片上传怎么将图片放到FileUpLoad中

页面代码：  <form id="form1" runat="server">
  <div>
  <br /><asp:TextBox ID="txt" runat="server" />    
  <div id="File">
  <asp:FileUpload ID="FileUpload1" onchange="txt.value=this.value"  runat="server" />
  </div>
  <asp:Button ID="Button1" runat="server" Text="Brow……" OnClientClick='document.getElementById("FileUpload1").click();' />   
  <br />
          <asp:Button ID="btnUpload" runat="server" Text="上传"  onclick="btnUpload_Click1" />
         <asp:Literal ID="lblMsg" runat="server"></asp:Literal>
  </div>
 </form>
后台代码：    protected void btnUpload_Click1(object sender, EventArgs e)
    {
        string a= FileUpload1.PostedFile.FileName;
        if (FileUpload1.PostedFile != null && FileUpload1.PostedFile.FileName != "" && FileUpload1.PostedFile.ContentLength != 0)
        {
            //判读图片的大小
            if (FileUpload1.PostedFile.ContentLength > 1024 * 600)
            {
                lblMsg.Text = "图片过大，上传失败";
                return;
            }
            //获得图片的后缀
            string sFilename = Path.GetExtension(FileUpload1.PostedFile.FileName).ToUpper();
            //.bmp .jpg .gif
            if (!(sFilename == ".BMP" || sFilename == ".JPG" || sFilename == ".GIF"))
            {
                lblMsg.Text = "图片的格式不正确";
                return;
            }
            //获得文件的名称
            string fileName = FileUpload1.FileName;
            string sNewfile = Path.GetExtension(FileUpload1.PostedFile.FileName);
            //保存文件的路径
            string pathName = Server.MapPath("~/Image/" + fileName);
            if (!Directory.Exists(Path.GetDirectoryName(pathName)))
            {
                Directory.CreateDirectory(Path.GetDirectoryName(pathName));
            }
            FileUpload1.PostedFile.SaveAs(pathName);
            if (File.Exists(pathName))
            {
                this.lblMsg.Text = String.Format("<a href='HotelImages/{0}'>Image/{0}</a>", FileUpload1.FileName);
            }
            else
            {
                //MessageBox.Show(this, "上传图片失败");
                lblMsg.Text = "上传图片失败";
            }
        }
    }   为什么我点击Button1的时候上传的图片 没有加到FileUpLoad中呢？ 请高人指点