Problemillas con una columna de plantilla de un datagrid

[Bravenap]

Bueno, esto ya está:

Código:

<asp:HyperLink id="hlDocumento" 
   ImageUrl='<%# DataBinder.Eval(Container.DataItem, "Extension") %>'
   NavigateUrl='<%# "~/verDocumento.aspx?id=" & DataBinder.Eval(Container.DataItem, "ID") %>'
   Runat="server" 
   OnDataBinding='CompruebaImagen'>

Y en el codebehind:

Código:

    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        If Not Page.IsPostBack Then
            Dim dir As System.IO.Directory
            Dim arrExt As New ArrayList
            Dim nombres() As String = dir.GetFiles(Server.MapPath("~/imagenes/iconos/"), "icoDoc*.gif")
            For Each nombre As String In nombres
                arrExt.Add(nombre.Replace(Server.MapPath("~/imagenes/iconos/") & "icoDoc", "").Replace(".gif", "").ToUpper)
            Next
            Session("extensiones") = arrExt

            ...

        End If

    End Sub

    ...

    Public Sub CompruebaImagen(ByVal sender As Object, ByVal e As EventArgs)

        Dim arrExt As ArrayList = Session("extensiones")
        CType(sender, HyperLink).Attributes.Add("title", "Archivo ." & CType(sender, HyperLink).ImageUrl)
        If arrExt.Contains(CType(sender, HyperLink).ImageUrl) Then
            CType(sender, HyperLink).ImageUrl = "~/imagenes/iconos/icoDoc" & CType(sender, HyperLink).ImageUrl & ".gif"
        Else
            CType(sender, HyperLink).ImageUrl = "~/imagenes/iconos/icoDocOtros.gif"
        End If

    End Sub

Un saludo.