File Explorer can be extended to support non-default functionality by means of Windows shell extensions, which are COM objects that plug the extended functionality into Windows Explorer. Shell extensions can be in the form of shell extension handlers, toolbars or even namespace extensions that allow certain folders (or even non-filesystem objects such as the images scanned by a scanner) to be presented as a special folder. File Explorer also allows metadata for files to be added as NTFS alternate data streams, separate from the data stream for the file.