There is only one file type that uses the .WEBZ extension:
WEBZ Compressed eBook File
eBook file used by Web Books Viewer, an eBook reader that runs in a Web browser; contains HTML files, images, and CSS files in a compressed .ZIP archive; decompressed and displayed when opened in the Web Books Viewer.
|Web Books Publishing Web Books Viewer — Discontinued||Free|
Updated: November 3, 2011