How to require FileUpload with actual upload, not just selecting a file

I have a FileUpload control along with a required field validator. It throws an error if the user d开发者_如何学Pythonoesn't click the Browse button to select a file (which is correct). However, if the user clicks the Browse button, but doesn't click the Upload button, ASP.NET's required validator doesn't throw an error. Any ideas how to fix?

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Why not use a CustomValidator instead of a RequiredFieldValidator?

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
    {
        args.IsValid = FileUpload1.PostedFile.ContentLength != 0;
    }

    private void Save()
    {
        if (Page.IsValid)
        {
            var myFileName = "somefile.jpg"
            FileUpload1.PostedFile.SaveAs(myFileName);
        }
    }