Here's one way you could do it...
Create a central routing handler specifically for grabbing files off the network and have your file servers named as sub domains pointing to your various file servers.
When a user clicks on the download link, e.g.
www.example.com/GetDownload.php?id=10
...the GetDownload.php page would look in the database to see where the file has been stored (assuming you're keeping track of the files locations in the database) or through whatever your convention is for keeping track of uploads, then determine the location of the file on your network. Then it could simply redirect the URL to the appropriate server/download folder. So GetDownload.php?id=10, upon finding the location of the file would redirect to the appropriate Server/URL:
AFile.doc is on FileServerB, redirect...
FileServerA.Example.com
Here! --> FileServerB.Example.com/A/AFile.doc
FileServerC.Example.com