validate mime types loaded from system files. Document that system files take precedence.

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-12-31.11:07:49.291>
labels = ['type-feature', 'library', '3.11']
title = 'validate mime types loaded from system files. Document that system files take precedence.'
updated_at = <Date 2022-01-18.13:39:10.730>
user = 'https://github.com/csabella'

bugs.python.org fields:

activity = <Date 2022-01-18.13:39:10.730>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-12-31.11:07:49.291>
creator = 'cheryl.sabella'
dependencies = []
files = []
hgrepos = []
issue_num = 32462
keywords = ['patch']
message_count = 4.0
messages = ['309275', '309291', '410864', '410865']
nosy_count = 3.0
nosy_names = ['r.david.murray', 'cheryl.sabella', 'iritkatriel']
pr_nums = ['5063']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32462'
versions = ['Python 3.11']