file-name-coding-system

file-name-coding-system is a variable defined in `C source code'.
Its value is

nil

Documentation:
Coding system for encoding file names.
If it is nil, `default-file-name-coding-system' (which see) is used.

On MS-Windows, the value of this variable is largely ignored if
`w32-unicode-filenames' (which see) is non-nil. Emacs on Windows
behaves as if file names were encoded in `utf-8'.