'Like Generator.__init__() except that an additional optional
argument is allowed.
Walks through all subparts of a message.  If the subpart is of main
type `text\', then it prints the decoded payload of the subpart.
Otherwise, fmt is a format string that is used instead of the message
payload.  fmt is expanded with the following keywords (in
%(keyword)s format):
type       : Full MIME type of the non-text part
maintype   : Main MIME type of the non-text part
subtype    : Sub-MIME type of the non-text part
filename   : Filename of the non-text part
description: Description associated with the non-text part
encoding   : Content transfer encoding of the non-text part
The default value for fmt is None, meaning
[Non-text (%(type)s) part of message omitted, filename %(filename)s]'

def __init__(self, outfp, mangle_from_=True, maxheaderlen=78, fmt=None):

    Generator.__init__(self, outfp, mangle_from_, maxheaderlen)
    if (fmt is None):
        self._fmt = _FMT
    else:
        self._fmt = fmt
