Arduino as File Server over wifi for file upload and download to SD card?

I am planning to use Arduino as a file server over wifi, which will upload file from a Webpage client(PC or mobile) to on-board SD card and also download the file from SD-card to the client PC or mobile that is connected to the wifi network. I am not looking at connecting to internet right now.

Do you have suggestions which arduino to buy and references to application that does this kind of work ?

Thanks, Pradeep

Don't use an Arduino for this purpose. An ARM-based Linux board (Raspberry Pi, Cubieboard, PCduino, etc.) will do that much easier and with already existing software that is proven in use for many years. The costs are probably less than an Arduino based solution and you get much more power and possibilities.