Retain FileUpload control value on PostBack in ASP.Net

Sir ,

I have a grid view which is costomized. I upload a image in this, when i click edit button then give error (Specified cast is not valid).

<td align="center" rowspan="11" style="width: 130px" valign="top">
    <asp:FileUpload ID="FileUpload4" runat="server" Width="100px" 
        Enabled='<%# bind("imagePath") %>' />
    <asp:Label ID="lblimage" runat="server" Text='<%# Bind("imagepath") %>' 
        Visible="False"></asp:Label>
</td>

please sir tell me how to solve this problem.