Go to System: Routing: Gateways and add an entry for the IP address of the remote router that your are connected to via OPT3. Do not check the default gateway box. Save.
Now go to System: Routing: Static Routes and add a route. Destination network is 192.168.4.0/24 (or whatever mask the fileserver uses) and the gateway is that you just created. Save.
If you did it right then pfsense now knows that any packet destined for the 192.168.4.0/24 subnet needs to be routed through the 10.0.0.x router at the other end of your wireless link.
Note that the router on the other end of the wireless link must also similarly have a route back to the networks connected to pfsense, using 10.0.0.253 as the gateway, unless you're using advanced outbound NAT to NAT everything out of OPT3, in which case the fileserver just thinks it's talking to 10.0.0.253, which the remote router already know how to find.
If the remote network (192.168.4.0/24)'s router (10.0.0.x) is using pfsense as it's default gateway then you can disregard the last paragraph, as it will route all non-local traffic to pfsense anyway.