Well I have to fix it as follow:
at /script/download.inc.php
there is a line of:
} elseif (preg_match("#(gif|jpg|png)#i",$file_extension)) {
I changed to :
} elseif (preg_match("#(zip|gz|rar|gif|jpg|png)#i",$file_extension)) {
now it works fine - the only difference is the browser does not get info how large the file is in total but downloaded file is not corrupted anymore.
Thanks for help guys ;-)
