Address Codex review feedback: only reject HTML responses when the file
metadata indicates a non-HTML type. Genuine HTML files (with mimetype
text/html or .html extension) are now allowed through.
- Check file.mimetype and file.name extension before rejecting HTML
- Add tests for legitimate HTML file downloads
Slack sometimes returns HTML login pages instead of binary media when
authentication fails or URLs expire. This change detects HTML responses
by checking content-type header and buffer content, then skips to the
next available file URL.
When replying to a Slack thread, files attached to the root message were
not being fetched. The existing `resolveSlackThreadStarter()` fetched the
root message text via `conversations.replies` but ignored the `files[]`
array in the response.
Changes:
- Add `files` to `SlackThreadStarter` type and extract from API response
- Download thread starter files when the reply message has no attachments
- Add verbose log for thread starter file hydration
Fixes issue where asking about a PDF in a thread reply would fail because
the model never received the file content from the root message.
