Agent mode does not persist `rendered_content` for user messages, so
when the next turn's history was rebuilt from DB, attachments uploaded
in prior turns disappeared — the model only saw the raw query plus the
prior assistant reply, breaking follow-up questions that referenced
the file (e.g. "what is in there?").
Reconstruct the attachment prompt from the stored `Attachments` column
when `RenderedContent` is empty, mirroring how image captions are
already replayed. KnowledgeQA turns (which do persist
`RenderedContent`) are unaffected and won't get attachments injected
twice.
Refs #1237
wizardchen
9ad8e7ca78