Thread:Project:Support desk/Can't upload non-image files - $wgFileExtensions set/reply (2)

I think in versions prior to 1.18, xlsx and odt files will be considered zip files (because technically they are). You may have to remove application/zip from $wgMimeTypeBlacklist as well.

Note, allowing arbitrary file uploads may open you up to some security issues, especially on pre 1.18 versions when allowing office formats