Manual:$wgUploadBaseUrl

Server URLs and file paths: $wgUploadBaseUrl
The base URL for uploaded images if shared upload directory isn't to be used
Introduced in version:1.4.0
Removed in version:Still in use
Allowed values:Unspecified
Default value:''

Details

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This can be used to set the server to use with $wgUploadPath if it is not the current server. However most people just put the full url in $wgUploadPath instead.

If set, this string is added to the start of $wgUploadPath to form a complete upload URL.