Whenever the Python launcher is selected, the installer sets the icon for common Python file extensions, i.e. .py, .pyc, .pyo, .pyw, .pyz and .pyzw. Formerly (under Python 3.4) the icons were located in DLLs in the Python installation directory. The new installer assigns either py.exe,1 or py.exe,2 as the icon. Because py.exe contains just a single icon, the only valid icon index is 0. As a result of the invalid icon, all Python files show up with either blank or generic icons. Probably those icons have not been included as they should have.

Tested on Windows 7 SP1, both X86 and AMD64, both fresh a fresh install and one with quite a bit of history. It occurs with all appropriate Python 3.5 installers (pick any of X86/AMD64 and web/non-web). Make sure the Python launcher is selected; the default settings will do.