The typical file upload tutorial is five lines: parse the form, get the file, save it to disk. It works for a homework assignment. It fails the moment someone uploads a 2GB file, or a .exe renamed to .jpg, or a thousand files simultaneously.
Production file handling needs three things the tutorials skip: size limits that protect your server, type validation that goes beyond the extension, and storage patterns that do not block your event loop.
Most Go developers know that Go uses goroutines for concurrency. A single process handles thousands of connections by scheduling goroutines across OS threads. That is usually enough.
But Fiber has a trick that most Go frameworks do not: prefork mode. One line of config, and Fiber spawns multiple OS processes, each with its own event loop, all listening on the same port. It is the same pattern that Nginx and Node.js cluster mode use.
The surprise is not that Fiber supports this. The surprise is when it actually helps - and when it makes your app slower.
You deploy your Fiber app behind Nginx. Everything works - until you check the logs. Every request comes from 127.0.0.1. Your rate limiter thinks all traffic is from one user. Your HTTPS redirect loop crashes the browser. And your geo-IP middleware thinks every visitor is in the same data center as your server.
The problem is three settings you did not configure. Fiber, by default, does not trust proxy headers - and it should not. But when you deploy behind a reverse proxy, you need to explicitly tell Fiber which headers to read and which proxies to trust.