How to use file upload control in asp.net

My Design part: <tr>
<td class="ControlCell" style="width: 90%">
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<contenttemplate>
<asp:FileUpload ID="FileUpload_PO" runat="server" Width="93%" ChildrenAsTriggers="false"/>

<asp:Button ID="btnAsyncUpload" style="display:none" runat="server" Text="Async_Upload" OnClick="btnAsyncUpload_Click"/>
<asp:Button ID="btnUpload" style="display:none" runat="server" Text="Upload" OnClick ="btnUpload_Click"/>

<Triggers>
<asp:AsyncPostBackTrigger ControlID = "btnAsyncUpload" EventName = "Click" />
<asp:PostBackTrigger ControlID = "btnSubmit" />
</Triggers>

<asp:RequiredFieldValidator ID="RequiredFieldValidator33" runat="server" ControlToValidate="FileUpload_PO" ToolTip="Select a file" Display ="Dynamic" ValidationGroup ="VgFileUpload_PO" >
<asp:Image ID="Image40" runat="server" ImageUrl="~/Images/Error.png" Width="20px" />

<asp:RegularExpressionValidator ID="RegularExpression_FileUpload_PO" ControlToValidate="FileUpload_PO" runat="server" ToolTip="" ValidationGroup="VgFileUpload_PO" Display="Dynamic" ValidationExpression="<%$appSettings:ValidUploadPOExtensionRegExp %>">
<asp:Image ID="Image20" runat="server" ImageUrl="~/Images/Error.png" Width="20px" />

</td>
</tr>
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Code Behind:
RegularExpression_FileUpload_PO.ToolTip = String.Format("{0}{1}{2}", "We only accept ", ConfigurationManager.AppSettings("ValidUploadPOExtension"), " file types.")

If lblPOUploadOriginalFilename.Text = "" Then
lnkbtnPOUploadLink.Visible = True
lnkbtnPORemoveLink.Visible = False
Else
lnkbtnPOUploadLink.Visible = False
lnkbtnPORemoveLink.Visible = True
End If
Select Case oMiscellaneousInvoice.BillToEditMode

Case clsMiscellaneousInvoice.BillToEditModes.Add
PnlCreditCard.Style.Add("display", "none")
PnlWire.Style.Add("display", "none")
PnlPO.Style.Add("display", "none")
PnlACH.Style.Add("display", "none")

PnlBillToEdit.Visible = True
ddlPaymentType.Visible = True
trDefault_CreditCard.Visible = oMiscellaneousInvoice.AllowSaveBillTo
trDefault_PO.Visible = oMiscellaneousInvoice.AllowSaveBillTo
trDefault_Wire.Visible = oMiscellaneousInvoice.AllowSaveBillTo
trDefault_ACH.Visible = oMiscellaneousInvoice.AllowSaveBillTo

BindDropDownListByValue(Me.ddlPaymentType, dtPaymentTypes, ddlPaymentType.SelectedValue.ToString)

If SameasSoldTo = False Then
BindDropDownListByText(Me.ddlBillToCountry, ViewState("dtCountries"), drBillToEdit("CountryID"))
If ddlBillToCountry.SelectedValue = hdnUSACountryID.Value Then
divBillToUSAddress.Style.Add("display", "b