feat(mobile): show S3-backed image attachment thumbnails (port #3707)

[Gilbert09]

Problem

Why: Image attachments on cloud task messages only appeared as file-name chips in the mobile app, making similar screenshots impossible to tell apart. This ports the desktop fix (#3707) to React Native.

Attachments also used to vanish after the task screen unmounted: the mobile app persisted a local echo of locally-sent attachments to AsyncStorage because the cloud log doesn't surface attachment data on user_message_chunk events. That cache never covered re-entry on a device that didn't send the message.

Changes

Image attachments on cloud task messages now render as real thumbnails, with the existing file chip as the fallback.

• Resolve from the log, not a cache. The session/prompt entries already in the S3-backed log carry the attachment URIs (file://…/.posthog/attachments/<runId>/<artifactId>/<file>). On snapshot load we parse those and reattach them to the historical user_message_chunk events, so thumbnails appear on re-entry and on any device — not just the one that sent them.
• Single source of truth. The local attachmentEchoStore (AsyncStorage) workaround is removed. In-flight optimistic rendering still works from the in-memory message the composer already pushes; the persisted cache it provided is now covered by log resolution.
• Presigned previews. Added presignTaskRunArtifact to the mobile HTTP layer and a useCloudAttachmentPreview react-query hook. The run's artifact manifest is fetched once per run through the shared query cache (a message with several images does one manifest request), and presigned URLs are cached with a stale time well under their expiry (50 min).
• Non-image attachments, and images whose preview fails to resolve, keep rendering as the file chip.
• HumanMessage stays a generic chat primitive: it exposes a renderAttachment seam so the tasks feature supplies cloud resolution, avoiding a chat → tasks import cycle.

Note: the artifact-URI parsing mirrors @posthog/core/sessions/promptContent; mobile deliberately does not depend on @posthog/core, so the two are kept in sync by hand (same pattern as the existing iconDomain mirror). A possible follow-up is to host the parsing in @posthog/shared, which both already depend on.

How did you test this?

• pnpm --filter @posthog/mobile test — 473 passing, including new unit tests for the artifact-URI parsing / reinjection (promptAttachments.test.ts, parameterised) and the presign API call (api.test.ts). The parsing tests cover the fallback-to-chip cases (ordinary file URIs, missing segments, non-image kinds).
• pnpm lint (Biome) clean on all changed files.
• tsc clean on the changed source files.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found 1 issue in 1 file · 1 warning.

1 warning

src/features/tasks/components/TaskSessionView.tsx

• ⚠️ src/features/tasks/components/TaskSessionView.tsx:800 Large component is hard to read and change no-giant-component

_{Reviewed by React Doctor for commit ee37418.}

[github-actions]

Contained mobile-only UI/data-resolution feature (image thumbnails from S3 artifacts) with no auth, billing, migration, dependency, or CI changes — it only calls an existing backend endpoint pattern via the established authedFetch client. Diff matches the description, includes solid new unit test coverage, and no reviews, holds, or unresolved comments block it.

• Author wrote 1% of the modified lines and has 26 merged PRs in these paths (familiarity MODERATE).

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	452L, 11F substantive, 710L/14F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1d-complex (710L, 14F, single-area, feat)
stamphog 2.0.0b3		.stamphog/policy.yml @ f057f43 · reviewed head ee37418