file can be a positional or keyword argument. Someone might be using it as a keyword argument and this would require a deprecation period before renaming could be done.

./python.exe
Python 3.9.0a0 (heads/master:7a68f8c28b, Jun 15 2019, 21:00:05)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import inspect, imghdr
>>> imghdr.what(file="README.rst")
>>> imghdr.what("README.rst")
>>> inspect.signature(imghdr.what).parameters['file'].kind.description
'positional or keyword'