Manual:$wgUploadNavigationUrl

Details
Point the " " link in the sidebar "  " section to a custom URL. Useful if you want to direct users to a shared repository by default without disabling local uploads, e.g.:

If you wish to point users to a custom upload page on the wiki, use an appropriate relative URL:

Red file links
As of   is used for red links. This allows pointing $wgUploadNavigationUrl to an external page that is not per se a Special:Upload directly. If is not defined, then $wgUploadNavigationUrl will be used as a fallback since.