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 $wgRemoteUploads to true to achieve this.

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

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 since.