The IDE categorizes all files by type. The Files tab displays the file types recognized by the IDE, the mime resolver for that type and the location of the file types listed as belonging to that type.
You can view all your existing file types by choosing Tools > Options from the main menu and clicking Miscellaneous in the Options window. Click Files and then set your file associations.
When selected, the IDE will scan the source code of projects to detect any files that were modified externally. Scanning is triggered when the focus is returned to the IDE, for example when the user returns to the IDE from another application.
If your sources are only modified from within the IDE, you can probably disable auto-scanning with little risk of encountering problems. Disabling auto-scanning may improve IDE performance if you are working on large projects.
Auto-scanning is enabled by default.
For more information about source scanning and other performance-related options, see the following FAQ on the NetBeans wiki.