A quick search: "BAK is a file extension for a file format typically used when creating a backup copy of a file. Normally, a BAK file is created as an automatic backup when an application is editing or making changes to the file in question."
On a Mac since 1984. Currently: 27" iMacs, Macbook Air, macOS 10.15.x,; iPhones, iPods and iPads galore!
Ira, I saw that on Wiki definition. But I've been using this software Interarchy 15 year. I've never seen bak files populate my website folder before. Are these harmless? Do the go away? Lot of clutter. Why all of sudden coming in? Was it the FTP client that made these or the Go Daddy server?