If the traffic is always initiated from the inside server, then all you need is a nat/global command.
You did mention however that files are copied in both directions, so if the DMZ server is initiating a file copy it would need to be as part of the same TCP/UDP connection, not sure how Windows would handle that though.
Easiest way to test it is to see if it works first, if it does then great. If not then enable syslogging on the PIX and look for deny messages from the DMZ server to the inside server, this will show you what type of ACL you need to apply to the dmz interface.