The File name filter is used to filter search results based on the path of the files and directories being crawled. There are several modes that are available to the filter:
Checking the box labeled Case sensitive, where applicable, will cause the path's string to be compared to its pattern in a case sensitive manner (case sensitivity is disabled by default). If case sensitivity is enabled, Help.txt will not match help.txt. It is rare that this feature will be used on NTFS/FATxx systems.
The File name filter can be disabled by un-checking the box to the far right of the label, just above the pattern text box.