It can be binary. In fact, the origin of flat files goes back to early data processing on mainframes. At that time there was no databases (as we know it today)

It can be binary. In fact, the origin of flat files goes back to early data processing on mainframes. At that time there was no databases (as we know it today) yet. It was a collection of records stored on punched cards, and loaded on tape (later disks) for processing sequentially. Records were not just text; they were a mix of binary, text, and BCD encoding.