Setting aside the Dropbox-proprietary services, I'd be most interested to see what features/categories you picked from the file metadata, how those were prepped

Setting aside the Dropbox-proprietary services, I'd be most interested to see what features/categories you picked from the file metadata, how those were prepped or rotated for training, what training/prediction algorithms you tried and how you picked a winner. Also curious to see the note that observing the "reason" was important here--isn't this a case where a total black box would be sufficient?