MIME Sniffing: feature or vulnerability?

This post describes a penetration-test finding where Internet Explorer's MIME sniffing rendered uploaded .zip/.csv files containing HTML/JavaScript as executable HTML, enabling persistent XSS; it explains how IIS's application/x-zip-compressed MIME type triggers IE's detection, contrasts behavior with Apache, and provides mitigations (X-Content-Type-Options: nosniff, Content-Disposition: attachment, whitelisting extensions, storing uploads outside web root, filename randomization) and recommendations for developers, administrators, and Microsoft.