Roger Richmond writes:

I have had a problem with Mailman stripping out MS Word documents, and have partially solved it by adding "application/msword" to the pass mime types section. However, Mailman still strips out documents created in Office 2007 and later, with the .docx extension.

The general approach is to look at the original mail containing the documents in a text editor (or Gmail's "Show Original") and search for the Content-Type headers. They'll look something like this:

Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document; name="some junk from the boss"

Most likely that's the one you want most. Add

application/vnd.openxmlformats-officedocument.wordprocessingml.document

to Pass Mime Types and you'll probably get most of them. However, not all mail programs conform to the registered types. There may be others you want to catch, but you'll have to investigate them yourself I suspect.