Assembling a file name

I am at it again
Shortened version of what I am doing, two tables...

1. Category which has a base file path (and udder stuff)
   
2. Files which (obviously) are files within the category.
   

Now what I do is upload files to the defined path (for the category) and now I want to add a file, in that table I select the category number, now what I would like to do is grab the path from the category table and do a list of files in that directory and select one to be this file.
So cat 1 - Utilities - path = /home/mydir/files/util

When I say add file, choose cat 1 I want a drop down so I can pick a file.
Make sense? It would be really neat if I could do this rather than enter the name manually we all know about speelos.
Thanks,

Larry