Namely, how would you tell an archive (jar/rar/etc.) file from a textual (xml/txt, encoding-independent) one?
There’s no guaranteed way, but here are a couple of possibilities:
Look for a header on the file. Unfortunately, headers are file-specific, so while you might be able to find out that it’s a RAR file, you won’t get the more generic answer of whether it’s text or binary.
Count the number of character vs. non-character types. Text files will be mostly alphabetical characters while binary files – especially compressed ones like rar, zip, and such – will tend to have bytes more evenly represented.
Look for a regularly repeating pattern of newlines.