[jQuery] Dynamic url in ajax call based on select value

[James]

I'm kind of scratching my head over this.  Okay, so let's say I want
to dynamically load a specific JSON flat file (could be xml, whatever)
depending on the current select box:

jQuery("#chained_sub_child").cascade("#chained_child",{
                                ajax: {url: $("select#chained").val() },
                                template: commonTemplate,
                                match: commonMatch
                        });

If the VALUE in the #chained select box was, let's say, "pizza",
jQuery would go ahead and attempt to load a file named "pizza".

But let's say I want to append an extension to "pizza".  For example I
want "pizza.json", or "pizza_1.js".

How do I get this extra string text into the ajax option above?

I have tried + notation, various combinations of brackets, etc.  But I
just can't get it to work and can't find appropriate documentation
anywhere on this.

Any ideas?

Thanks,
James