Microsoft has told me they can't do anything, Trello said:
Our team is still looking into this, and since the integration is owned by Microsoft, our integrations manager will reach out to Microsoft for further investigation. In the meantime, I'd recommend using the board email address to add cards (https://support.atlassian.com/trello/docs/creating-cards-by-email/), and/or manually add attachments. I will keep you posted on further updates.