Archives compressed with zlib usually require writing a custom unpacker using the zlib library . Once you get the individual data files, you should figure which types of files are there. If the file names have extensions, the file type can be usually determined from the extension. If they don't, you can try looking at the beginning of the file and seeing whether there is a text signature. After you get the files sorted into separate formats, you can start deciphering individual data formats.
Text files are usually the simplest ones to figure out.
Even if they don't, they usually have some keywords or other clues to what data is contained in the file. Examples of text files in Infinity Engine games are the IDS, 2DA, INI files and the compiled AI scripts. Chunked files contain a sequence of individual pieces - chunks . Each chunk is prefixed with a small header containing the type and length of the chunk and probably some other data. None of the Infinity Engine files have that structure.