> What, then, is your recommended solution which generally blocks .zip files > but allows the xlsx, docx and pptx file suffixes?
It was not my intention to recommend something. Because file(1) and amavis are not perfect, banned_* is a wish list. As you have already observed, the outcome deviates from the wishes right now, and I pointed out that it still might deviate from them using your proposed whitelisting. I have no suggestion how to align expressed wishes with the outcome in all corner cases. That said, a recommendation would then mean to make you change wishes, even I they were justified or originated from a third party you have no control over. Personally, I do not block archives.
