Manual:$wgHashedSharedUploadDirectory/de-formal

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   to.