add filename_extension_map and/or content-types_map dict(s) to mimetypes

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-30.21:18:32.903>
labels = ['3.8', 'type-feature', 'library', '3.9']
title = 'add filename_extension_map and/or content-types_map dict(s) to mimetypes'
updated_at = <Date 2019-04-04.04:01:00.975>
user = 'https://bugs.python.org/DanielBlack'

bugs.python.org fields:

activity = <Date 2019-04-04.04:01:00.975>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-03-30.21:18:32.903>
creator = 'Daniel Black'
dependencies = []
files = []
hgrepos = []
issue_num = 36489
keywords = []
message_count = 4.0
messages = ['339221', '339388', '339417', '339431']
nosy_count = 3.0
nosy_names = ['r.david.murray', 'xtreak', 'Daniel Black']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36489'
versions = ['Python 3.8', 'Python 3.9']