Thanks...
in fact, the new files ".tar" are not valid archive for 
PowerArchiver and the ".tgz" files are not open correctly 
(as they are .tar files in fact-  Right?).
So I renamed one ".tgz" to ".tar" and it works great !

Anyway, I Still don't understand why filetypes in Windows 
OS are defined by extensions :)
(And why powerarchiver doesn't improve that)