[issue39324] Add mimetype for extension .md (markdown)

[Irit Katriel]

Change by Irit Katriel <iritkatriel at gmail.com>:


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versions: +Python 3.11 -Python 3.6, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39324>
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