WIP - download external payloads

[rossedfort]

Description & motivation 💭

Adds download functionality for externally stored payloads (note: only for payloads that display in a Code Block). Other payload touch points will be addressed in a following PR.

WIP
This PR is a work in progress. Currently the download button shows, but the download functionality doesn't work.

Screenshots (if applicable) 📸

Design Considerations 🎨

Testing 🧪

How was this tested 👻

• Manual testing
• E2E tests added
• Unit tests added

Steps for others to test: 🚶🏽‍♂️🚶🏽‍♀️

1. Check out this branch and run pnpm i and pnpm build:server and pnpm dev
2. Clone Justin's fork of sdk-go which contains an external payload workflow and codec server
3. Follow the instructions in the notion doc titled: External Storage Bug Bash 04/17
4. Set the AWS_PROFILE and S3_BUCKET environment variables
5. In your local version of Justin's fork of temporalio/sdk-go, run go run demos/worker/main.go, and in a separate shell session, run go run demos/codecserver/main.go
6. Run temporal workflow start --task-queue "echo-task-queue" --type "GenerateDocument" --workflow-id "generate-doc" --input '1000000' to run a workflow with an externally stored payload.

Checklists

Draft Checklist

Merge Checklist

Issue(s) closed

Docs

Any docs updates needed?

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Project	Deployment	Actions	Updated (UTC)
holocene	Ready	Preview, Comment	May 4, 2026 8:58pm