Minimal changes to handle Slack API changes to files
Minimal changes to handle Slack API changes to files

Per https://api.slack.com/changelog/2018-05-file-threads-soon-tread `file` is
now `files` (it claims the format is different, but for our purposes the same
struct still works).

mihaip/slack-archive,mihaip/slack-archive,mihaip/slack-archive,mihaip/slack-archive