FileUpload control within UpdatePanel in asp.net

My source file is:

 

 

 

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.