I will try to describe the behavior on linux:

(Format used is "what i type in file name" -> "name which gets written to disk")

1. When files of type is: Python Files
   a -> a.py
   b.py -> b.py
   c.py.py -> c.py.py
   d.py.abc -> d.py.abc (!)
   e.abc -> e.abc (!)

2. When files of type is: text file
    Same behavior as before, with .txt instead of .py/.pyw

3. When files of type is: All files
   a -> a
   b.py -> b.py
   c.py.py -> c.py.py
   d.py.abc -> d.py.abc
   e.abc -> e.abc

I hopefully have tried most of the combinations. If I have left out any, please let me know and I'll try them out.