project_release.add_file can't upload binary data

If I try to add a binary file to a project release

f=release.add_file(filename='dummy', file_content='\xff', content_type='application/octet-stream')

I get
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/dist-packages/launchpadlib/resource.py", line 337, in __call__
    args[key] = simplejson.dumps(value, cls=DatetimeJSONEncoder)
  File "/var/lib/python-support/python2.6/simplejson/__init__.py", line 243, in dumps
    **kw).encode(obj)
  File "/var/lib/python-support/python2.6/simplejson/encoder.py", line 360, in encode
    return encode_basestring_ascii(o)
UnicodeDecodeError: 'utf8' codec can't decode byte 0xff in position 0: unexpected code byte

because json cannot encode binary data (unless a special encoding such as base64 is used). Is there a way around this or am I doing something completely wrong?

Python 2.6.2 on a default jaunty system:
ii python 2.6.2-0ubuntu1
ii python-launchpadlib 0.2~bzr35-0ubuntu1
ii python-simplejson 1.9.2-1build1
ii python-wadllib 0.1~bzr7-0ubuntu4