Ok, I solved it with the unvaluable help of a nice guy in the #python
channel.
It is a studid as it gets, replaced
files = [('fileupload', 'b.jpg', open('b.jpg').read())]
by
files = [('fileupload', 'b.jpg', open('b.jpg', 'rb').read())]
because binary files are not opened correctly in Windows XP unless you
specify that they are so.