Design a file storage and sync service (like Dropbox).

Split files into chunks (e.g. 4 MB) and hash each chunk; upload only changed chunks (deduplication, resumable uploads).
Chunks in object storage (S3); a metadata database stores files, versions, chunk lists and permissions.
Upload directly to storage with pre-signed URLs.
Sync: clients keep a cursor; a notification service (long polling / WebSocket) announces changes; clients pull new metadata.
Conflicts: keep both copies, or version history.
Sharing with ACLs and links; CDN for downloads; encryption at rest.