content-type text/plain has file extension .ksh?

Python 2.7:

>>> from mimetypes import guess_extension
>>> guess_extension('text/plain')
'.ksh'

Python 3.5:

>>> from mimetypes import guess_extension
>>> guess_extension('text/plain')
'.c'

How can I get a valid answer?

For me ".txt" would fit.

Even the filetype lib can't handle this :-(

See https://github.com/h2non/filetype.py/issues/30

from content-type text/plain has file extension .ksh?