Unfortunately, on Windows Vista machines, any .ZIP files will not be shown for selection.

This relates to Vista’s ability to consider .ZIP files as folders into which a user can navigate within Windows Explorer.  This means that they don’t appear with the file list for the multiple upload.

There are some registry-change methods available to work around this but I find the quickest solution to be this:

• Rename any .ZIP file to another extension prior to upload, e.g. .ZIP2
• Select and upload the file in the multiple file upload selection
• Use the Open with Windows Explorer option under the Document Library’s Action menu to browse to the uploaded file on SharePoint and rename it back to .ZIP.

This may not sound like a great solution and you may wonder why one would bother, especially for a single file.  Using the Multiple File method, allows the user to see a progress bar for the upload.  Additionally, the single file upload method may time out with a large single .ZIP file.

Not ideal, but it gets the job done!

Silverlight extensions for SharePoint

Can re-use full Silverlight XAML in full WPF application on desktop