I used to prepare wills for a living but I'm unaware of any strict legal definition of what is meant by 'contents' in this context. There's certainly no relevant statute law and there would only be applicable case law if the question had actually been tested before a court.
I would expect that, in the absence of anything in the will to the contrary, a court would rule that 'personal possessions' (such as jewellery) form part of the contents of a house. However I very much doubt that a car would be regarded in the same way.
A court would be likely to take a 'common sense' approach. For example the court might consider what an auctioneer would expect to collect if he was asked to clear the contents of a house. In the absence of any other instructions he'd almost certainly take jewellery but (unless he was specifically instructed to do so) it's very unlikely that he'd take a car.
Chris