It seems that there is a list of files from which the mime types are also added at https://github.com/python/cpython/blob/5c0c325453a175350e3c18ebb10cc10c37f9595c/Lib/mimetypes.py#L42. "video/x-matroska" is not present in CPython repo's list of suffixes so it should be inferring from the list of known files. Can you please run the below script on 3.7.4 and 3.7.5 on the same machine? I am using Mac and 3.7.4 and 3.7.5 report video/x-matroska correctly.

import mimetypes
print(mimetypes.guess_type('E01.mkv'))
print(mimetypes.types_map['.mkv'])