File Upload Handling

This handles the full lifecycle of file uploads: validation, sanitization, virus scanning, storage, and serving. You get reference implementations for Flask, FastAPI, and Express with Multer, plus S3 integration patterns. The examples cover the security essentials like MIME type validation, secure filename generation, and directory traversal prevention. Most useful when you're building upload features from scratch and want to avoid the common security pitfalls. The emphasis on signed URLs and CDN integration shows it's thinking beyond just accepting files. Solid starting point if you need more than a basic multipart form handler.