Expose attachment filename and MIME type from page listings

[Flerpharos]

Summary
Expose attachment filename and MIME metadata already present in page entry listings before payload download.

Scope

• retain parsed attachment metadata on the listing-side object model
• make filename and MIME type accessible without fetching attachment content
• keep payload download behavior unchanged

Acceptance criteria

• attachment listing objects expose filename and MIME type
• metadata access does not trigger payload download
• metadata-first workflows can use the exposed fields directly

[Flerpharos]

Design note: AttachmentEntry.filename and .mime_type should be populated only from the page-listing response. They are listing metadata, not a second mutable attachment cache. Attachment download and update behavior should remain unchanged; a refreshed page obtains a new listing.