Default or Plain Text Filter
Indexing Service is no longer supported as of Windows XP and is unavailable for use as of Windows 8. Instead, use Windows Search for client side search and Microsoft Search Server Express for server side search.
For plain-text files and files of unregistered origin, Indexing Service uses the default filter (in Query.dll). It filters both the system properties (such as file names) and the contents of a file. The default filter does not “understand” any document formats; when filtering the contents of a file, it treats the file as a sequence of characters. (It does check for the Unicode byte-order mark at the beginning of the file.) Indexing Service uses the default filter when the file-name extension of a file has no association in the registry, and if the value of the FilterFilesWithUnknownExtensions registry entry is one.