FileUpload控件不能上传文件

我把一个FileUpLoad控件放在一个Ajax的UpdatePanel控中就不能上传文件了.这是怎么回事?有什么解决的办法吗?
aspx文件源代码:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default4.aspx.cs" Inherits="admin_Default4" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>无标题页</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
    
    </div>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
                <asp:FileUpload ID="FileUpload1" runat="server" />
            </ContentTemplate>
        </asp:UpdatePanel>
    </form>
</body>
</html>
cs文件码:
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;public partial class admin_Default4 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string path = "~/upload/Image";
        FileUpload1.SaveAs(path + FileUpload1.FileName);
    }
}