FileUpload control within UpdatePanel in asp.net

Question

My source file is:

 

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="Button1" runat="server" Text="Upload" onclick="Button1_Click" />
</ContentTemplate>
</asp:UpdatePanel> 

 

and code behind code is:

 

protected void Button1_Click(object sender, EventArgs e)
{
if (FileUpload1.HasFile)
{
string Prefix = DateTime.Now.ToString("ddMMyyhhmmsstt");
string Doc_Name = (Path.GetFileName(FileUpload1.PostedFile.FileName)).Split('.')[0].ToString();
string str = FileUpload1.FileName;
string targetPath = Server.MapPath("~\\images\\Docs\\") + Prefix + FileUpload1.FileName;
FileUpload1.SaveAs(targetPath);
}
}

 

 My problem is that FileUpload1.HasFile return false, why ?

And how can fix it and save the file in folder.

Please help me...

Thanks in advance.