mimetypes.guess_extension unable to get non-lowercase mimetype

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-24.09:50:19.046>
labels = ['type-bug', '3.9']
title = 'mimetypes.guess_extension unable to get non-lowercase mimetype'
updated_at = <Date 2022-01-15.17:46:52.069>
user = 'https://github.com/inkhey'

bugs.python.org fields:

activity = <Date 2022-01-15.17:46:52.069>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2020-01-24.09:50:19.046>
creator = 'Inkhey'
dependencies = []
files = []
hgrepos = []
issue_num = 39441
keywords = []
message_count = 2.0
messages = ['360601', '410653']
nosy_count = 2.0
nosy_names = ['Inkhey', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39441'
versions = ['Python 3.9']