Go to 'Items displayed'.
Uncheck all in two first columns except for 'Custom files'. Enter some extension in Extensions input field (TXT). Go to some directory with files with such extensions. They are not displayed.
You need to check also 'Text files' to display them. There is misconception here:
'Custom files' should define exceptions from above options, so that provided extensions are displayed or not according to 'Custom files' setting. Currently 'most restrictive' setting is used on extensions conflict. I firmly believe that 'Custom files' should have precedence over other options for the reasons given.
X.
XnView 1.82 <x>
rc2: another 'Items displayed' problem
Moderators: helmut, XnTriq, xnview