Hi Wize Owl,
Unfortunately, filtering the extensions is not possible due to the way browsers implement the <input type='file' />. This is browsers limitation, not RadUpload one. Since RadUpload relays on the stanrad input type = file, it inherits this fault.
The only way to have filtering, multiple file selection and other useful features, is to use Flash or Silverlight upload. Having said that, you might find this demo
useful. It shows how to use Silverlight upload in an ASP.NET environment.
the Telerik team