In case this does not get fixed for a long time, here is a work around (re-implement the encoder yourself):


.
.
.
def myencoder(msg):
  from base64 import encodebytes as _bencode

  orig = msg.get_payload()
  encdata = str(_bencode(orig), 'ascii')
  msg.set_payload(encdata)
  msg['Content-Transfer-Encoding'] = 'base64'
.
.
.
# here is an example of how to use it
fp = open('/usr/share/openclipart/png/animals/mammals/happy_monkey_benji_park_01.png', 'rb')

bindata = fp.read()

x = email.mime.image.MIMEImage(bindata, _subtype='png', _encoder=myencoder)

y = x.get_payload()
print (y)
.
.
.