[docs] [issue12798] Update mimetypes documentation

[Roundup Robot]

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

New changeset cfa3fe9d7b1f by Senthil Kumaran in branch '2.7':
porting mimetype doc changes from 3.2.
http://hg.python.org/cpython/rev/cfa3fe9d7b1f

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