Why

Companion to #142 (backend file.attach handler + contract 2). The client half of dbbd1d4d0 was reverted by the same blocked-upstream-sync split-brain: on main the attachment-sync path only handled images, so a non-image @file: ref sent to a remote gateway resolved to a client-disk path the gateway can't see ("path outside the allowed workspace"). This restores the client upload path so remote sessions (the Phase 2b cross-device feature) can actually attach files.

What changed

• apps/desktop/src/app/session/hooks/use-prompt-actions.ts:
  • syncImageAttachmentsForSubmit → syncAttachmentsForSubmit: images keep image.attach / image.attach_bytes; non-image files now use the file.attach RPC (upload bytes when remote, pass the path locally). Restores readFileDataUrlForAttach + the image-bytes helpers that were also clobbered.
  • rewriteOptimistic refreshes the optimistic message after refs resolve.
• Merged against current main, not a blind take-theirs: kept the Phase 2b sender_device on prompt.submit; the remote check is $connection.get()?.mode === 'remote' || activeBackendIsRemote() so it fires for both whole-app remote mode AND a Phase 2b dialed-endpoint session; rewriteOptimistic carries the same interrupted: false fresh-turn invariant as seedOptimistic.

How to review

1. syncAttachmentsForSubmit — the image branch is behavior-identical to before; the new file branch is the restored feature.
2. The combined remote condition + the rewriteOptimistic interrupt reset (the two main-divergence merge points).

Evidence

• uploads file bytes via file.attach on a remote gateway and passes the path directly via file.attach in local mode now pass; clears a leftover interrupted flag on a fresh submit passes.

Verification

• tsc -b 0 errors. vitest use-prompt-actions.test.tsx — 15/16 pass. Full vitest src — 469 passed / 7 failed; all 7 are pre-existing on clean origin/main (the 6 in untouched files + the 1 below).

Risks / gaps

• One pre-existing failure remains in this file: sleep/wake "session not found" resume-retry. It fails identically on clean origin/main, is unrelated to attachments (the resume-retry escape hatch returns ok:false), and is filed separately as a pre-existing-failure investigation (possibly another clobbered half of the upstream-sync split-brain).
• 2 use-gateway-boot test failures persist on main (remote reconnect escape hatch) — pre-existing, flagged for a separate look (possible fallout from feat(desktop): attach to sessions on other devices — Live section + remote dial (Phase 2b/2) #139's prune keep-set); not touched here.
• End-to-end remote file upload needs a two-device setup to exercise live; tracked with the rest of the channel canary on F-003-multi-participant-channels.

Collaborators

• @OmarB97 (operator)
• Claude Fable 5 (Claude Code)