Extension checks vs MIME sniffing vs content inspection

These checks sit at different depths.

Extension checks

• Fast and useful as a first filter.
• Easy to bypass because filenames are user-controlled.

MIME sniffing

• Looks at bytes or signatures instead of trusting the request header.
• Better than extension checks, but still only answers what the file appears to be.

Content inspection

• Looks for risky structures or matches inside the file.
• Needed for PDFs, archives, macro containers, SVG, and more nuanced threat patterns.

Practical takeaway

Use all three in the right order:

1. Extension allowlist.
2. MIME or magic-byte validation.
3. Structural inspection or optional signature scanning.

Continue

• Why extension checks are not enough
• File-type validation vs malware scanning
• Secure file uploads in Node.js: Beyond extension and MIME checks
• Defense in depth for file uploads