Still https://github.com/python/cpython/blob/3a1d50e7e573efb577714146bed5c03b9c95f466/Lib/imghdr.py#L11 signature is def what(file, h=None).

I 'd like to suggest to update it into def what(filename, h=None)
Also, the doc of imghdr represents it as the filename.
https://docs.python.org/3/library/imghdr.html

note that sndhdr is already using this name.
https://github.com/python/cpython/blob/3a1d50e7e573efb577714146bed5c03b9c95f466/Lib/sndhdr.py#L52

If this proposal is accepted, I'd like to send a patch for it.

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'

Inlike to sndhdr.what(), imghdr.what() accepts not only file names, by file objects theirself. The feature was added at 1997 in 45ac47c0b2798cc3a29cf5111694aba5e9347e08.