By "early days", I was more thinking about the 1999-2005 era, before low-end Nokias even got Bluetooth and the ability to play MP3s.

The wireless headset was the killer app that drove bluetooth adoption within cellphones, driving down costs until eventually the lower-end models receiving it too. While sharing files was possible in the 1999-2005 era (especially with PDAs), most phones were lacking enough flash storage to store anything worthwhile.

While I don't want to say file sharing wasn't a killer app, it does seem to have been limited to just schools during a certain time period.

A time period that I missed out on by a few years. At high school, we did all our file sharing by swapping burned CDs. Then we switched to dragging around laptops and USB hard drives at university (and using the private emule network on the university wired ethernet).