File Upload Handling

Help me implement file uploads for {{use_case}} in {{stack}}. Cover validating file type and size on the server (not just the client), virus and malicious-content concerns, where to store files (object storage, not the app server), uploading large files (chunking, direct-to-storage signed URLs), progress feedback, generating thumbnails or processing async, access control on retrieval, and cleanup. Note the security risks of file uploads (malware, oversized files, path traversal, executing uploaded content) and how to handle them. Keep it secure and scalable.