I agree, it's contempt of court. As well it should be, since the court is contemptible. The right against self-incrimination is absolute - you don't have to testify against yourself, you don't have to unlock that (combination) safe, you don't have to decrypt files. You have the right to remain silent.
That is, unless it's the physical key to a safe, or some hardware encryption key. That's physical, and subject to seizure. But a combination or encryption password is a product of the mind, and forcing it out is forcing self-incrimination.
Sure, law enforcement has a right, with the proper warrant, to break into the safe or attempt to decrypt the contents themselves, but failing that, they're simply SOL.