For some reason, the set_file method is not available for ListControls file upload objects inside the Browser object. To be able to fill in the file field, one needs to do

myfilecontrol.mech_control.set_file(filestream, mimetype, filename).

The problem gets weirder as set_file() is the method indicated by the README.txt doctest of zope.testbrowser package as the way to upload a file stream in a file upload widget, and I presume the README.txt test doesn't fail.