Manual:$wgUploadNavigationUrl

Details
Point the upload navigation link to an external URL. Useful if you want to use a shared repository by default without disabling local uploads, e. g.:

It is not necessary to set Manual:$wgRemoteUploads to true to achieve this.

Red file links
As of r69997 $wgUploadMissingFileUrl is used for red links. This allows pointing $wgUploadNavigationUrl to an external page that is not per se a Special:Upload directly. If $wgUploadMissingFileUrl is not defined, then $UploadNavigationUrl will be used as a fallback.