This option shows only the files whose name matches the entered file name mask. The entered mask may consist of multiple file masks seperated by semi-colons.
You can also specify to use by mask by checking the Name mask option and entering the file name mask on the Options | Preferences - Thumbnail View settings tab.
Example: *.jpg;dog*.bmp;*_doc.tif This would show all .jpg files, any .bmp files starting with 'dog', and any .tif files ending with '_doc'.
See also: Options | Which Files | Selected Types.