OnServerValidate function is not being called in custom validator.

protected void yourButton_Click(Object sender, EventArgs e)
{
    Page.Validate();
    if(Page.IsValid)
    {
       // write your code here
    }
}

<form id="form1" runat="server">
    <div>
        <asp:FileUpload ID="fuCheatingEvidence" runat="server" Width="890px" Size="50" />
        <asp:CustomValidator ID="cvFileUpload" runat="server" ErrorMessage="Please select file!"
            ControlToValidate="fuCheatingEvidence" OnServerValidate="cvFileUpload_ServerValidate"
            ValidationGroup="vgSubmitForm" ValidateEmptyText="True"></asp:CustomValidator>
            <asp:Button ID="bthUpload" runat="server" Text="Upload" ValidationGroup="vgSubmitForm" OnClick="bthUpload_Click" />
    </div>
    </form>

protected void bthUpload_Click(object sender, EventArgs e)
   {
       //your code for uploading
   }

   protected void cvFileUpload_ServerValidate(object source, ServerValidateEventArgs args)
   {
       String fileName = fuCheatingEvidence.PostedFile.FileName;

       if (fileName != "")
       {
           args.IsValid = true;

       }
       else
       {
           args.IsValid = false;
       }

   }