MIME Type Checker

How the signatures work

Most binary formats begin with fixed bytes, often called magic numbers. PNG starts 89 50 4E 47, JPEG FF D8 FF, PDF %PDF, and gzip 1F 8B. The browser-reported type usually comes from the file extension alone, so a renamed file shows a mismatch between the two lines.

Many modern formats are ZIP archives inside: DOCX, XLSX, PPTX, EPUB, JAR, and APK all start with ZIP's PK bytes, so a correct result for them is application/zip. Text formats such as SVG, CSV, and JSON have no signature at all.

Checking types on upload

An upload form that trusts the extension or the browser's reported type can be fooled by renaming a file. Check the leading bytes on the server, store files under names you generate, and serve them with an explicit Content-Type plus X-Content-Type-Options: nosniff, so browsers do not guess a different type. To confirm a file has not changed since it was checked, record its file hash.

How to use it

1. Choose a file up to 50 MB.
2. Inspect detected and reported types.
3. Compare the result with the extension.

Privacy & limitations

Only local bytes are inspected. Detection is focused, not an exhaustive forensic identification system.