There are no errors in robots.txt
The problematic attachment pages are not blocked / not giving problems by robots.txt
However, guests have no permission to view attachments which may be related. I'm not sure why Google is trying to index pages which have no permission anymore. (they did in the past) It does make sense to me that Google indexed these pages in the past, even with the noindex tag. The purpose is most likely not to index those pages, but to evaluate those for relevance.
I have asked the host to enable fileinfo.