ASP.NET : How to use customvalidator with a file upload tool for client side validation

We really do not want to validate in the server side because it makes the server busy.
So a good approach is to validate in the client side using javascript. Here is a good tutorial that does it:

http://www.beansoftware.com/ASP.NET-FAQ/FileUpload-Validation.aspx

** I believe when there are so many quality tutorials over the web, I do not really need to write one of my own unless the topic is something unique**