If all else fails, when trying to extract from some archive you know little about I would try installing BSD Tar (it is readily available on most distros) and using that. It uses libarchive and can extract from a wide range of formats, tar, cpio, ar, zip, rpm, ISO_9660, etc. and automatically detects a variety of compression methods. It supports most of the same command line options as GNU tar. So to inspect the contents of an archive:
For a more detailed overview with a summary showing archive format and compression at the end:
bsdtar tvvf archive
Or to extract
If this doesn't work you could always try to gather more information about what the file actually is using the file command:
and if that doesn't work search on the internet using the file extension as one of your keywords.
That all said, when asking questions it is better to include what your 'end goal' is as it may allow someone to suggest an alternative method to achieve what you desire.
: I only just realised this was a dead thread that was resurrected by baloon68. Sorry about that!