to allow non-networked computers to access network files stored on the SD card
If the computer is not connected to a network, how is it going to access the Arduino via it's ethernet shield?
Or, is the ethernet part of the shield simply excess baggage?
What is a network file? A file is a file, whether it is located on the PC, on a share that the PC has access to, on an external device, like a USB memory stick, etc.