About Masks
A mask is a kind of filter through which filenames are passed to determine whether they can be displayed or not. Masks are not case sensitive. The asterisk character (*) is used to denote multiple characters, so the mask *.* displays all files, since all filenames have some characters, a dot and an extension. The ? character replaces single characters, so the mask do?.txt would allow the display of files named dog.txt, DOT.txt, and so on.

You can combine masks to create one longer mask allowing more filenames though by separating each with a semi-colon (;). For instance, the mask *.txt;*.doc;*.h?? would allow all files with .txt and .doc extensions though, along with files whose extensions start with an 'h' and consist of 3 characters.

Masks are entered by typing them into the Mask field on the toolbar. Press Enter to apply the mask. The Mask utility attempts to be intelligent about this: if it thinks you should have a dot or an asterisk in the mask, it adds it for you (so you can view all text files by typing just txt rather than the unwieldy *.txt). If you want to prevent the mask being changed like this, press Shift+Enter to apply the mask unaltered.