mimetypes.guess_extension unable to get non-lowercase mimetype

[7dbc2e6c-091a-47dc-b62e-98b63be23712]

BPO	39441
Nosy	@inkhey, @iritkatriel

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['type-bug', '3.9'] title = 'mimetypes.guess_extension unable to get non-lowercase mimetype' updated_at = user = 'https://github.com/inkhey' ``` bugs.python.org fields: ```python activity = actor = 'iritkatriel' assignee = 'none' closed = False closed_date = None closer = None components = [] creation = 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'] ```

[7dbc2e6c-091a-47dc-b62e-98b63be23712]

mimetypes.guess_extension and mimetypes.guess_all_extensions doesn't work correctly with non-lowercase mimetype.

>>> import mimetypes
>>> mimetypes.guess_type('file.pptm')
('application/vnd.ms-powerpoint.presentation.macroEnabled.12', None)
>>> mimetypes.guess_extension("application/vnd.ms powerpoint.presentation.macroEnabled.12")

>>

This issue exist because we automatically convert type as lower in guess_all_extensions, but we do not prevent added type to be lowercase.

[iritkatriel]

See also bpo-20392.