Manual:$wgHashedUploadDirectory

Details
If, new subdirectory is created in   folder (e.g.  ) for each file upload. If, all images are uploaded in /images folder itself.


 * This parameter should not be changed after the first image or file has been uploaded (or you need to move all previous uploaded pictures). It will result in php-errors (see the discussion page) if articles with the old logic try to find their pictures, since the image path is calculated and not stored in the database.