What requirements did nobody mention?
Answer it out loud before you open anything. The value of the flags below is in comparing them to what you actually said — including whether you asked who uses it before naming what you would build it with.
A brief for a file-upload service: "Users upload documents from the web app; other users download them. Max file size is 50 MB (illustrative). Use S3 or similar." That is the whole brief. You are asked to estimate it by Friday.
React to this
Say what you would question, what you would keep, and what you would need to know first.
Clarifying questions a candidate sent back (illustrative): 1. Which cloud provider do you prefer? 2. Should I use presigned URLs or proxy through the API? 3. Do you want a progress bar? 4. Node or Go for the service? 5. Should thumbnails be generated? Not asked: who may download whose files; what happens when an upload is interrupted; how long files are kept; whether the same file may be uploaded twice; whether a file can be replaced; whether uploads must be scanned before they are downloadable.
What it is really testing
Whether the candidate knows that a brief lists what the author thought of, and that the hard requirements are usually the ones nobody wrote down: who may download what, what happens to a half-uploaded file, whether uploads must be scanned, how long files are kept, and what "download" means for a file that is being overwritten.