If you have your file names in a database, then you first need to create a record set to pull the names from the database. Then you will need to create a drop down menu that lists all your files (make sure to set the value to the name of your file). And then apply Digital File Pro Download Server behavior and set the file name to the select list.
I have attached a file with an example that could show you an example.
Let me know if you have any further questions.