Edited
Having looked over the manual , there is no way. The wildcards are used to find not exclude files found. The function reads all none-empty files into separate strings. *.mld or ?layer.mdl both are ways of finding files.


EDIT2 -
Thoughts on achieving the goal - In many cases the file extension name(char set) is not important. So save.txt and save.txx can both be text files and if a text editor opens and looks at them, it will read it perfectly as a text file. This can be used to sort text files. Like so
txt_for_dir(tDir,"*.txt");
txt_for_dir(tDir,"*.txx");