Added support for FileEntities to downloadEntity and uploadFile

Note: I left in place a small bug that's present in downloadEntity for both old locationable entities and the new FileEntities. The issue is that adding entries for 'files' and 'cacheDir' into entities causes an error if you then call updateEntity. The platform doesn't like those keys, 'cause they're not part of the JSON schemas. I figure we can fix that when we move to representing entities in Python as an Entity object, rather than a dictionary as presently.