[docs] [issue12798] Update mimetypes documentation

[Roundup Robot]

Roundup Robot <devnull at psf.upfronthosting.co.za> added the comment:

New changeset 9c19df6c8ea0 by Senthil Kumaran in branch '3.2':
News entry for Issue12798
http://hg.python.org/cpython/rev/9c19df6c8ea0

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<http://bugs.python.org/issue12798>
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