### What?
This PR ensures that bulk uploads fail if any file is missing, rather
than skipping missing files and proceeding with the upload.
### Why?
This fixes unintended behavior where missing files were skipped,
allowing partial uploads when they shouldn't be allowed.
### How?
- Prevents submission if any file is missing by checking `req.status ===
400`.
- Updates `FileSidebar` to correctly handle cases where a file is
`null`.