Manual:$wgRemoteUploads

Files and file uploads: $wgRemoteUploads
Set to true to enable the upload link while local uploads are disabled.
Introduced in version:1.2.0
Removed in version:1.17.0 (r74627)
Allowed values:Unspecified
Default value:false

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Set to true to enable the upload link while local uploads are disabled. Assumes that the special page link will be bounced to another server where uploads do work.

Compare also Manual:$wgUploadNavigationUrl which may be the better solution in many cases.