Nearly all archives now have file extenders:
xxx.spk: Spark archive
xxx.zip: Zip archive

Other archive types:
xxx.Z: compress
xxx.gz: GNU-Zip (gzip)
xxx.tar: tar

Text files:
xxx.txt, xxx.info, README

If a file doesn't have an extender, it is either a text file or an
unrecognized archive (those archives should disappear as soon as I
have some time for this :)
 
Please note that most README's don't have the extenders in the
descriptions!