You can specify file types (by using their file name extensions) to include in the content index when crawling content sources. For example, you might want to include files that have .abc and .def extensions in the index.
This list only controls what the search system will attempt to crawl. If there is no filter installed for that document type, the indexer will not be able to read its contents, although it will be able to get the basic metadata such as file name from the file system that holds it. For more information about managing file types and using filters, see the article "Limit or increase the quantity of content that is crawled " on SharePoint Server 2007 on TechNet.
To manage file types, you must first open the Manage File Types page:
On the Search Administration page, under Crawling, click File types.
What do you want to do?
Add a file type
On the Manage File Types page, click New File Type.
On the Add File Type page, in the File extension box in the File Name Extension section, type the file name extension for the file type that you want to add (for example, type doc).
Delete a file type
On the Manage File Types page, in the list of file name extensions, click Delete on the menu for the extension of the file type that you want to delete.
In the message box, click OK to confirm that you want to delete the file type.