Whenu you are configuring the 'if file exists' portion if you choose to rename the file you can swap out what is specified as the increment for your own value:
[NewFileName]_[Increment]
So if you had your new value that you wanted to use in a variable you could put that in here in place of the increment. You could also just append another value after this that will help you to keep the filename unique. For example if you specified the id of a record you are working with after the increment you would append that value to the file name as well. It might look like this:
php:
[NewFileName]_[Increment]<?php echo $_SESSION['your variable'] ?>
