I think this is more of a webserver configuration issue rather than a concrete5 specific problem.

It comes down to MIMETYPES. These are set up on the server itself so that it understands how to serve up different kinds of files. I'm thinking that your server simply doesn't have mimetypes set up for docx, xlsx, pptx, etc. When it doesn't have a mimetype for a file, I think it just looks at the file itself and makes a guess. The new microsoft document formats ARE actually zip files, so that would explain why they are downloading that way.

Some webserver administration panels (like cPanel) allow you to set up new mimetypes. Sometimes you need to add entries to an .htaccess file, and sometimes you might need to contact your host to add them.

Have a look at the first and second posts of this forum thread:
http://www.webdeveloper.com/forum/showthread.php?threadid=162526...

The second post in particular is the most useful - adding those entries to your .htaccess file is likely to do the trick.