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.:

It is not necessary to set to true to achieve this. If you wish to point users to a custom upload page on the wiki, use an appropriate relative URL:

If $wgUploadNavigationUrl is set to false, then a link to Special:Upload is added to the toolbar if uploads are enabled and the user has both upload and edit permissions. If $wgUploadNavigationUrl is set to a url, the link is always shown, regardless of the user's permissions.

Red file links
As of r69997 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 $UploadNavigationUrl will be used as a fallback since.