problem of gettting null value in fileupload control during postback of page in updatepanel control

Question

hi...........
i have a problem in accessing filename in uploadfile control in asp.net. During postback of any button the filename becomes null. I use Fileupload.attribute["value"] property but the dat becomes null. How can i maintain filename during postback of page.
 protected void cboPageNo_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (UP_Logo.HasAttributes || UP_Logo.HasFile) <-- up_load contain null value on this event
        {
            UP_Logo.Attributes["values"] = UP_Logo.PostedFile.FileName.ToString();
        }
        multView.ActiveViewIndex = 1;
        ViewState["Content"] = cboPageNo.SelectedItem.Text;
    }

please give me the reply soon.
thanks a lot...........