When I try to upload a file that contains a question mark on it, it displays an error, prompting me to change the target filename:

Invalid character(s): %s. Please try again

But the wiki accepts it as a destination filename. The problem is that it's listed as a forbidden character:

# FIXME: these 2 belong somewhere else, presumably in family
# forbidden characters are handled by pywikibot/page.py
forbidden = ':*?/\\'  # to be extended

I guess it could be removed, since if it's really forbidden, the wiki will throw the error when trying to upload.