ASP.NET Web窗体自定义验证器不会触发

<asp:FileUpload ID=\"fuVendorBrief\" runat=\"server\" />
<br />
<asp:CustomValidator ID=\"cvVendorBriefFile\" Display=\"Dynamic\" runat=\"server\" ValidationGroup=\"EditorValidate\" ControlToValidate=\"fuVendorBrief\" OnServerValidate=\"cvVendorBriefFile_ServerValidate\" ErrorMessage=\"You must upload a vendor brief PDF file.\">     
</asp:CustomValidator>

<asp:Button ID=\"btnSubmit\" ValidationGroup=\"EditorValidate\" OnClick=\"btnSubmit_Click\" runat=\"server\" Text=\"Add Vendor Brief\" />

protected void cvVendorBriefFile_ServerValidate(object source, ServerValidateEventArgs args)
{
    CustomValidator fileUploadValidator = (CustomValidator)source;
    FileUpload vendorBriefFileUpload = (FileUpload)fileUploadValidator.Parent.FindControl(fileUploadValidator.ControlToValidate);
    args.IsValid = vendorBriefFileUpload.HasFile && vendorBriefFileUpload.FileName.ToLower().EndsWith(\".pdf\");
}