FileObjects have attributes. These are ad-hoc key-value pairs which can be associated with a file and persist across shutdown and restart.
Attributes are routinely used in the System Filesystem for providing additional configuration data. In pre-4.0 NetBeans, they were relatively commonly used for user files on disk. They still may be used for user files, but this is now discouraged - the infrastructure has been changed to store all attributes in the userdir rather than sprinking .nbattrs throughout the users disk; using attributes for user files at this point does have negative performance implications.
Source: NetBeans FAQ