Entropy doesn't make much sense for a single instance. There trivially exists a codec that will translate any particular file into any other. To get a real idea how much information is in a file you'll need some statistical model about how it may have been generated (thankfully the model doesn't have to be perfect).

You will also want to be a bit careful what assumptions you allow this model to make. In a very real sense a hash is mostly enough information to recover a file if you know it exists somewhere, as torrents and magnet links prove.