Slack inbound events with subtype=file_share were silently dropped, so
nanobot never saw messages that included attachments. Allow file_share
through, download Slack-private files using the bot token into the
local media dir, and pass them to the agent as media paths plus a
"[file: name]" / "[image: name]" placeholder in the content. Reject
responses that look like Slack's login HTML so an auth page is never
saved as if it were the user's file. Document the required files:read
scope alongside files:write so installs that read attachments are not
quietly missing the permission.
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