Manual:$wgImgAuthUrlPathMap

Details
For   requests, everything after   is checked to see if starts with any of the prefixes defined here. The prefixes should not overlap. The prefix that matches has a corresponding storage path, which the rest of the URL is assumed to be relative to. The file at that path (or a 404) is send to the client. Example:

The above maps   to  . The name  should correspond by name to an entry in .