Jules of Diner's Room gives a quick but thorough lesson in French law concerning sexual assault and slanderous allegations. My layman's reading is that the charge of attempted rape is likely to be thrown out because, even on Banon's account, there was no attempted penetration, so the alleged acts, if they took place, should be classified as agression sexuelle, on which the statute of limitations has run. But the criminal charge could lay the basis for a later civil charge, which would still be receivable.
Second, it's not clear why DSK's lawyers filed a counter-charge of dénonciation calomnieuse rather than diffamation.
Not being an expert in French law, I may not have this exactly right, but I'm sure Jules will correct me if I've misrepresented him. In any case, I'm grateful for a very illuminating post.