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 .