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   folder itself, see also .

Set this setting to false especially if you have a set of files that need to be accessible by all wikis, and you do not want to use the hash (path/a/aa/) directory layout.

Setting this to true is equivalent to setting the   property in <tvar|ForeignFileRepos> </> to <tvar|2> </>.