ASP.NET file upload

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ASP.NET

C#5.0

I have web form where I need to upload a file. I have set the maximum file size size as 10mb in web.config file. Now I need to validate the file upload in the form. If I try to upload a file size greater then 10mb it is moving to HTTP Error 404.13 - Not Found page but instead I need to display a error message in the same form as The maximum file has been extended . I used the following code for that purpose.

protected void btnSave_OnClick(object sender, EventArgs e)
   {
       if (uxFileUpload.FileBytes.Length > 0 )
       {
           int iMaxFileSize = 2097152;
           if (Request.ContentLength > iMaxFileSize)
           {
               exceedErrormessage.Visible=true;

               //Server.Transfer("Error.aspx");

           }

<div class="file-upload">
        <asp:Label ID="uiFileUpload" runat="server" AssociatedControlID="uxFileUpload" Text="Select file" />
            <p>The maximun file size is 20mb</p>
        <asp:FileUpload ID="uxFileUpload" runat="server" />
            <asp:PlaceHolder ID="exceedErrormessage" Visible="false" runat="server">
                 <asp:Literal ID="exServerErroemessage" runat="server" Text="The maximum file limit has been exceeded" />
    </asp:PlaceHolder>
            <%--<asp:Label ID="label" runat="server"></asp:Label>--%>
            <asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="uxFileUpload"
   ErrorMessage="File size should not be greater than 20 mb." OnServerValidate="btnSave_OnClick"></asp:CustomValidator>
        <asp:Button ID="btnSaveDocument" ValidationGroup="SaveFile" OnClick="btnSave_OnClick" runat="server" Text="Upload" CssClass="button" />
    </div>

But still it is moving to the same error page when I try to upload a file size greater then 10mb. I even tried placing a breakpoint and checked where the problem is occurring. I found that when I try to upload a file size greater than 10 mb it is not even moving to the btnSave_OnClick function but otherwise if I upload a file size less than 10 mb it is moving to the function and the corresponding operation is been performed. Can anyone help me to solve this problem.