Filename filters only apply to unindexed searches. 

IncludeFilters and ExcludeFilters can each contain one or more filename filters, separated by spaces, to apply to files in the directories selected. (ExcludeFilters can be blank.) If a filename filter contains a space, put it in quotation marks. A filename filter that does not contain a backslash is compared to the name of each file. A filename filter that contains a backslash is compared to the fully- qualified pathname of each file. Example:  

*.DOC *DOCS*.WPD "*My Documents*"

ISearchJob Interface