Hi, I'm not sure if this would be considered a bug or a feature request, but IDLE's file-completion is case-sensitive in Windows (e.g. if I have a file named ABBA and I type in 'abb' + tab, ABBA won't pop up. Since Windows files systems are not case-sensitive, it might make sense for the file-completion to disregard case as well. I'm not sure if this behaviour is preferred, however, so I'll leave that up for discussion.
I tested the behaviour on 3.3 and 3.4; I assume it's the same for other versions as well.