Manual:$wgHashedSharedUploadDirectory

Details
If you operate multiple wikis, you can define a shared upload path using  .

If   is set, the wiki will look in the shared repository if no file of the given name is found in the local repository (for, [[Media:..]] links).

If  , new subdirectory is created in   folder (e.g. ) for each file upload.

If  , all images are uploaded in   property in   to  .