When there is an XML file head ("<?xml version="1.0" encoding="UTF-8" standalone="yes"?>") in a file attached to an email - and even if the file extension is txt - the content filtering fails to find a text/regex pattern within the file...
the tech support case# is 09397278
It should work as expected, there is no point in undocumented omiting files from content filtering
Regards
P.