I didn't see anything specific in the rule book, but we played with the discard pile face up. We also played so that you could look through the discard pile at any time to see how many cards were discarded for different sets, assessing the red velvet special action, etc.
But God shows his love for us in that while we were still sinners, Christ died for us.
In your world, I have another name. You must learn to know me by it. That was the very reason why you were brought to Narnia, that by knowing me here for a little, you may know me better there.
The rules don't specify, but face up makes the most sense to me, and is always how we've played it. Cards such as the eclair or day-old donuts become far more calculateable and strategic when the discard is face up, rather than a random chance. In any case, the only benefit to a face down discard comes to those with a stellar memory. Personally I'd play it face up whatever an official ruling calls because it's just more fun that way. The moment you take a light game like this one and create unnecessary limits to the gameplay, you lose some players' interest who would otherwise enjoy it for the light strategic game that it is.