Do you actually have to remove the *.txt extension from scan exclusions or can you just uncheck the scan on “Execute” box for *.txt files? It seems to me that what you will then have is a universal scan on “Execution” with a list of exclusions. So by unchecking the “Execute” box for *.txt files the execution of a *.txt file would longer be considered an “Exclusion” and would therefore be scanned by Avast.