I think there are two types of communication we need to distinguish. Pre-file, the communication we do before gettting the file, and post-file, the communication we do post file.
Pre-file we can communicate as much as we want without being penalized. We can say for instance that we want our bitstring to be interpreted as an elf binary to be run in ubuntu 14, with specific libraries installed. Or we can say that it will be a zip file. Or we can say that it should be pre-concatenated with 0's. We can specify hundreds of thousands of lines of code that should be compiled now, and then later be executed with the bitstring as input.
Then we get the file, and now every bit we communicate is penalized. If we have asked for a turing-complete format we may now send a decoder specific for the file.
