Here you may reduce (filter) the file(s) you want to search based on file's wildcards or mask.
You may enter several wildcards, separated by a semicolon ( ; ).
Example:
to search all documents with extension .DOC and spreadsheets with extensions .XLS,
should type in File Mask box:
*.DOC; *.XLS
You may review most recent used (searched) masks clicking the down arrow on the File Mask(s) box.