[lifthrasiir]

> Maintaining a static list of the most common incompressible types within Dropbox and doing constant time checks against it in order to decide if we want to compress blocks

There is also a format-agnostic and adaptable heuristic to stop compression if the initial part (say, first 1MB) of the file seems incompressible. I'm not sure whether this is widespread, but I've seen at least one software doing that and it worked well. This can be combined with other kinds of heuristics like entropy estimation.