File mime type checker

What Is a MIME Type?

MIME types (image/png, application/pdf) tell systems what kind of file they are handling — browsers use them to decide whether to display or download, servers to set correct headers. This tool detects a file's true type from its content, not its extension.

Why Extensions Lie

Renaming virus.exe to vacation.jpg does not change its bytes — content-based detection sees through the disguise. Useful for validating uploads, debugging "wrong file type" errors, and basic security screening.

How to Use the File MIME Type Checker

Detect a file’s true type from its signature bytes — magic numbers — instead of its extension, so you can verify uploads and catch disguised files before they reach your server.

1. Upload the file or drag it into the checker window.
2. The tool reads the first bytes and matches them against a database of known file signatures.
3. Compare the detected MIME type, such as image/png, with the file’s extension — a mismatch is a red flag.

Example

A user uploads “photo.jpg”, but the checker detects application/x-dosexec — the MZ header of a Windows executable. The extension lied, and your validator rejects the file before it can do harm.

Frequently Asked Questions

Why is the extension not enough?
Extensions are just filename text — anyone can rename virus.exe to cute.jpg. Magic bytes are written by the program that created the file and are far harder to fake convincingly.

What are magic numbers?
Fixed byte sequences at a file’s start that identify its format: PNG files begin with 89 50 4E 47, PDFs with %PDF, ZIP archives with PK. The checker matches these signatures to name the real type.

Is this enough for upload security alone?
No — treat it as one layer. Combine signature checks with extension allowlists, size limits, antivirus scanning, and a separate upload domain to contain anything that slips through.