Issue 6799: mimetypes does not give canonical extension for guess_extension with text/plain

>>> import mimetypes

>>> mimetypes.guess_extension("text/plain")
'.ksh'

most others are correct, it is just this one is quite wrong. I would
recommend changing it to .txt . 

>>> mimetypes.guess_all_extensions("text/plain")
['.ksh', '.pl', '.c', '.bat', '.h', '.txt', '.asc', '.text', '.pot', '.brf']