TL;DR HTML file upload security is not just about adding accept="image/*" and calling it a day. Most beginners ship file inputs that are wide open to abuse — cat photos named resume.pdf , multi-gigabyte server bombs, and worse. This guide covers the 5 fixes that actually matter, but the most dangerous mistake is one almost nobody talks about until it is too late. The Problem: Your File Input Is a Open Door Here is a scenario that has happened to more developers than will admit it. You build a resume upload form. It goes live. On Monday morning you open the uploads folder and find 37 JPEG files all named resume.pdf . No actual resumes. Just chaos. That is not bad luck. That is an unguarded <input type="file"> doing exactly what it was built to do — accepting anything from anyone. HTML file upload security is one of those topics that looks simple on the surface and turns into a disaster the moment real users get involved. The good news?…