If it's being overwritten, it's probably because of a rewrite that they have set up... if your base URL is correct, then the /data directory should be available via that... the one thing I can think of is if on the production server they are using a NAS or a different location to store the attachments, but that should not really need a rewrite as you can do a symlink
for the /data directory to a different storage device and then configure nginx to allow access to that location outside the root. It's what I currently do as I have a 100GB bucket in addition to my regular Digital Ocean VPS, and I store all the attachments and such out on it.