It is actually a common error on the web with the new 2007 office
formats. I think it has to do with the way mime types work over the
internet, but all of the Office 2007 files convert to .zip files. The
reason why they come out as .zip, is because the files are actually
a .zip file with the various .xml and other files that make up the
document combined in a certain structrure that describes the format.
You can simply rename the .zip file with the correct extension and it
will work fine.
-DZero