fetchMessageAttachments downloads a Gmail message's downloadable parts as
discord.js file descriptors, skipping parts over Discord's 25 MB ceiling and
capping at 10 files per message. Nameless inline parts (CID screenshots) get a
synthesized name; nameless text/* parts (the email body Gmail serves as an
attachmentId-backed part) are skipped. The poll posts these on both new tickets
and follow-ups, naming any skipped parts so staff know to check Gmail.