There wasn't a change in the file Extension. It was still a .QBW file (thus the ability to open it with ease, as a normal file and view the data as if nothing changed).

Basically what happened was is that it created TWO files when you wanted to archive (after choosing the date at which you wanted to cut off transactions), it would created a .QBB (backup file of the current file) then it would create a .QBW file and RE-name it with "Archive ..." Infront of the name of your current file.

When you opened the file, that file name would show up at the very top of the QBD program (just as it does now, as the company name... with Archive in front of it)...

The original file would have the old Data split off (or rather Condensed) from it, but the name would be retained the same.