Server.MapPath() to set URL path to asp.net server

I am trying to create a function to upload files to a folder 'files_uploaded' in the root of my website, using FileUpload and Server.MapPath("~/files_uploaded/") to set URL paths to asp.net Server.
The below code works on my development machine using MS development server, but not when uploaded to my website server.
I have tried many variations for MapPath Tilde. Can anyone suggest where I am going wrong?
 
<script type="text/vbscript" runat="server">
Sub btnAdd_Click(ByVal sender As Object, ByVal e As EventArgs)
If (upImage.HasFile) Then
If (CheckFileType(upImage.FileName)) Then
Dim filePath As String = ("~/files_uploaded/") & upImage.FileName
upImage.SaveAs(MapPath(filePath))
End If
End If
End Sub
Function CheckFileType(ByVal fileName As String) As Boolean
Dim ext As String = Path.GetExtension(fileName)
Select Case ext.ToLower()