If it’s truly a “card,” it makes sense to file behind it. If it’s a hanging folder with a label, and that label is at the back of the folder, it makes sense to file in front of the label, in the folder. Could that be where the confusion lies?
I agree with Musicat–my file cabinets at work don’t have alphabetical dividers, just hanging folders labeled as whatever, in which case the things being filed go in front of their label. But if it was something like a card catalogue my gut would be to file after the card.