Manual:$wgHashedSharedUploadDirectory

Details
If you operate multiple wikis, you can define a shared upload path using $wgSharedUploadPath. If $wgUseSharedUploads 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 /images folder itself, see also Manual:$wgHashedUploadDirectory.

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.