are you transferring the powertap data directly from the power tap to the USB stick, or via another device?
real time bike power data is transferred according to the Bike Power device profile (refer to device profiles in the adopter zone).
FIT data is stored data ad may be transferred wirelessly using ANT-FS or via USB cable depending on your device.
stored ANT+ data may be included in a FIT file. specifically, ANT+ power data is likely included in an FIT activity file