I have a cardreader connected to USB port and a Mac with gpg installed (obviously).
GPG itself from the command line works with cardreader perfectly - it see it, can read keys, can decrypt, can do anything.
GPGFileTool fails on a file that can be succesfully decrypted with the command line gpg. It does something with the cardreader, the cardreader blinks, and then throw a message "decryption fails".
Maybe it happens because of PIN request? Have someone tested GPGFIleTool with keys stored on a smartcard?