User blog comment:Sarah Manley/ Product Update - Video/@comment-188432-20120320150228/@comment-188432-20120320155342

It appears it's not just us using question marks in file names. There's also:
 * starwars:File:Darth Revan ?.jpg
 * memoryalpha:File:How Much for Just the Planet?.jpg
 * w:c:icarly:File:?!?!.jpg
 * w:c:wowwiki:File:Harpies?.png

So this relatively widespread use of question marks in filenames indicates that if you intend to add question marks to, say,, you need to make a separate, prominent announcement and send out message notifications to all users about it. I think the fact that a number of communities that don't share an awful lot of common users are using question marks in filenames is evidence enough that we don't know we're not supposed to be using question marks in filenames.

Luckily, not that many filenames on any of these wikis include question marks. So if you want to make the change, it won't be so hard. But, again, you do need to view it as a change — not something that your user base already knows about.