[feature] Upload sample document for actions with FileEntry output

As a workflow designer
I want to upload a sample file as an output
So that I can reliably test the next steps in the workflow

https://share.cleanshot.com/M6sbYPJ3

No files or tests are named. Start by tracing how workflow actions represent FileEntry outputs and how sample outputs are currently configured; the screenshot may clarify the intended upload interaction. Done means a designer can upload a sample document as an action output and use it reliably in subsequent workflow steps.

Written by the indexing model from the issue text.