Random python mimetypes detections

See https://github.com/python/cpython/commit/9fc720e5e4f772598013ea48a3f0d22b2b6b04fa#r45794801 and this [Python bug](https://bugs.python.org/msg384730)

The changes introduced by this ticket in https://github.com/python/cpython/commit/9fc720e5e4f772598013ea48a3f0d22b2b6b04fa#r45794801 are problematic.

I discovered this from having tests failing when testing on Python 3.7 and up

The bug is that calling mimetypes.init(files) will NOT use my files, but instead use both my files and knownfiles.
This was not the case before as knownfiles would be ignored as expected when I provide my of files list.

This is a breaking API change IMHO and introduces a buggy unstability : even if I want to ignore knownfiles by providing my list of of files, knownfiles will always be added and this results in erratic and buggy behaviour as the content of "knownfiles" is completely random based on the OS version and else.

The code I am using is here https://github.com/nexB/typecode/blob/ba07c04d23441d3469dc5de911376d408514ebd8/src/typecode/contenttype.py#L308