IE7 Changes file extensions when downloading certain file types

When downloading ".tgz" files from internet sites, IE7 automatically changes the file extension to ".gz". The file can be changed back without corruption, but what a pain. Does anyone know how to stop IE7 from changing the file extension in the first place. I've tried registering the ".tgz" file type and associating it with various zip programs, nothing seems to work. Windows accepts the file type and knows what to do with it, just IE7 won't allow that file extension to be transferred through without changing it.