Perhaps you can use javascript to make your pre-populated drop-down lists visible/hidden,
e.g.
1. by default, your form offers drop-down list #1, CSS properties of drop-down list #2 are to hide it
2. user can select from drop-down #1
3. on the form, there's an add button
4. user clicks on the add button -> calls some java to change the CSS properties of drop-down #2 to display
Now, I guess you can repeat that for the number of files you're going to allow to be added? Otherwise I'm not too sure how you go about doing this without going off to another/same script with parameters.
Anyway, hope that helps.
Jim.....
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