Attachments support & validation (images/files) in chat

[mehalyna]

Task 8 — Attachments support & validation (images/files) in chat

Title: attachments: allow attaching uploads in messages and safe serving

Description

• Support attachments via:

  • Pre-upload to existing POST /api/uploads/ (recommended) and include upload_id in message payload.
  • Server resolves upload_id and stores reference in message document attachments.

• Validate attachment types and size server-side; reject disallowed types.

• Serve attachment URLs via signed URLs or public S3 links (reuse Uploads service).

• Tests for image attachment and reject invalid file.

Acceptance

• Messages created with attachments show attachment metadata and URLs in responses.
• Clients can download attachments using returned URLs.

Dependencies

• Uploads service (existing).
• Task 3 message service persists attachment references.

Notes

• For inline images show thumbnail url vs full url.