Move the MIME/type-to-extension maps and derivation helpers out of
session-manager.ts into a dedicated attachment-naming module — keeps
session-manager focused on session lifecycle and gives the helpers
a natural home for unit tests alongside the existing attachment-safety
module.
Two small fixes alongside the extraction:
- extForMime now guards `typeof mime !== 'string'` before .split, so a
buggy bridge passing `mimeType: { ... }` (object) no longer crashes
the inbound write loop.
- deriveAttachmentName computes Date.now() once per call instead of
twice, and tightens the explicit-name check to a string-and-truthy
guard so non-string values fall through to derivation.
Adds attachment-naming.test.ts with 11 cases covering MIME normalization
(case + parameters), Telegram type fallback, the non-string defensive
guard, and the bare-timestamp fallback.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
