Overriding builtin error messages

[Steve Swinsburg <s....@lancaster.ac.uk>]

I'd like to override the default form feedback messages (also so I can  
localise them). ie

'Upload must be less than'

In Form.java I found this :
// Resource key should be <form-id>.uploadTooLarge to
// override default message

But I'm still not sure how to override it in a Properties file if the  
form-id is dynamic?

Any help much appreciated.

Steve