File Uploads

Handles the unglamorous but critical parts of file uploads that break in production. Validates files by magic bytes instead of trusting extensions, generates safe filenames to prevent path traversal, and enforces size limits before your server runs out of memory. Covers S3 and Cloudflare R2 with presigned URLs for direct uploads, plus multipart handling for large files. The sharp edges section is genuinely useful, walking through real attack vectors like malware.exe renamed to image.jpg and how attackers exploit cached presigned URLs. If you're adding file uploads to anything and don't want to learn these lessons the hard way, this is a solid reference.