@ aimee: It's mentioned in the full length version of the featurette Arlette posted (I finally got around to watching it
). The costume designer mentions that the skirt is made of linen, but he doesn't say what the bodice is made of.
Speaking of which, I just got back from Joann's and got some gorgeous blue linen for my Belle cosplay. It took a while to find a good shade, but I'm so glad I found it!