Equipment to play video reels would be pretty easy to reconstruct when given a single reel. Even a VHS would be much harder, and reverse engineering the JPEG specifications would be nigh impossible.
That is assuming you have to reverse engineer it. While it is true that some proprietary formats will inevitably be lost to time, a lot of open formats could be reconstructed in the future simply by preserving the format specification. Think ogg or mkv for example.