I am a little puzzled by this issue.
1. Why would a Windows user use any extention other than .py or .pyw, which IDLE opens fine. How many people do something senseless like hand-registering, say 'xiq', as a python file?
2. .pyc files are binary files and NOT source code files and cannot be opened by IDLE as it will attempt to decode them into text. So why the check for that?

Also, I believe there is another issue to add a 'Treat this as Python code' entry to one of the menus. This would cover the case of files without extensions and files that are not python files but happen (on a case-by-case basis) to have python code embedded within.

So I am thinking that this should be closed.