Power was right. Unfortunately(or not) I found his answer after I spend half an hour debugging the webtest. Here is a bit more information.
Trying to pass only the path to the files brings you the following exception:
webtest/app.py", line 1028, in _get_file_info
ValueError: upload_files need to be a list of tuples of (fieldname, filename, filecontent) or (fieldname, filename); you gave: ...
The problem is that is doesn't told you that it automatically will append the field name to the tuple send and making 3 item tuple into 4 item one. The final solutions was:
avatar = ('avatar',
file(os.path.join(settings.PROJECT_PATH, '....', 'avatar.png')).read())
Too bad that there is not decent example but I hope this will help anyone else too )