Bistro de Paris is a great restaurant; we enjoyed it so much more than we did Chefs de France. It is generally quieter upstairs, with a nice, romantic atmosphere. Most people were dressed up a bit - women in dresses or nice slacks, men in collared shirts. I didn't see anyone in a jacket there, but they were generally a bit dressier than just park wear.
Because it's in the theme park, though, be prepared for a few elements that don't quite fit in. There was one woman there wearing a bikini top, whose baby screamed nonstop. I can't imagine wearing a bikini with a sarong to a restaurant of this caliber and somewhat upscale atmosphere, but apparently the staff let her in, b/c there she was! They looked like they'd just strolled in from Typhoon Lagoon.
There were also a few more kids here than we noticed at the nicer restaurants located in resorts. One family sitting near us let their kids run all around - they blew out candles on other tables, and were under the server's feet as he tried to prepare crepes suzette, which could have been a dangerous situation.
Cheers!
Heather W