Using older versions of Outlook for Mac for personal email and calendar
It's been a month or so and haven't lost an attachment, Yay. Not sure what changed, but appears to be working as intended.
However, I will not update the Macbook software in fear that it will start dropping attachments all over again.
I've been using the old Outlook for Mac for the past few months because of this problem (it doesn't suffer from this problem), but I'd like to go back to the new one some day. Until Microsoft confirms its fixed though, I'm reluctant to suffer the humiliation of looking like a moron to my customers, as Ross mentioned.