How to use MIME Type and File Signature Checker

Choose up to 20 files, or type file names or extensions in the lookup field. You can use both in one run.
Press Inspect file signatures.
Read each row: what the bytes look like, what the extension says the file is, and the verdict between them.
Open the notes when a row is a ZIP container, because that family shares its opening bytes with several formats.

Example: MIME Type and File Signature Checker

Check a correctly named PNG, the same PNG renamed to .jpg, and a PDF.

You add

Files: logo.png holding PNG bytes, photo.jpg holding those same PNG bytes, and report.pdf beginning with %PDF-1.7.

You get

The page reports three files, two matched and one contradicted. logo.png and report.pdf are listed as matches, and photo.jpg is listed as a mismatch because the bytes look like a PNG image while the extension names a JPEG.

Options

Extension lookup: Type names or extensions such as report.docx or .png, separated by commas or spaces. The page lists their recorded MIME types without opening a file. A lookup describes a name; it cannot verify any file bytes.
What the bytes look like: The first bytes are compared with a reviewed list of signatures, from PNG and JPEG through PDF, GZIP and SQLite to the MP4 family. RIFF containers are told apart by the marker inside them, and the MP4 family by its brand.
Extension expects: The extension is looked up in a table of the type and MIME name it normally means. This is what the name claims, not what the bytes proved, and the two are printed next to each other so the difference is visible.
ZIP containers: DOCX, XLSX, PPTX, ODT, EPUB and JAR files all begin with the same ZIP bytes. The report reads the first stored entry name as extra evidence and then names the family from the extension, because the bytes alone cannot.

Supported inputs and limits

Look up at most 40 names or extensions within 4,000 characters, or inspect up to 20 files. The opening 4,096 bytes of each file are compared with the signature list, and a ZIP container gets one extra read of up to 64 KB to find its first entry name; nothing else is read and nothing is uploaded. A text file has no signature, so printable UTF-8 text is reported as text with lower confidence, and a file whose bytes match nothing reviewed is reported as not recognised. Renaming a file never changes its bytes. This page compares signatures only: it does not scan for malware, does not unpack an archive, and cannot tell you whether a file is safe to open.

Where your input is processed

This tool processes your input in this browser. Your text and files are not uploaded to UseFreeTools. Check this tool's limits for anything it may save on your device.

What a file signature can and cannot prove

Signature bytes are a claim by whoever wrote the file, not a guarantee. A damaged download can keep its opening bytes while the rest is broken, and a file can be built with one format's header and another format's payload. Use the signature to catch a wrong extension or a mislabelled upload, and keep the real checks for size, structure and origin with the system that consumes the file.

Questions about MIME Type and File Signature Checker

Why does a DOCX come back as a ZIP container?

Because it is one. Word, Excel, PowerPoint, OpenDocument and EPUB files are all ZIP archives with a particular set of entries inside, so the shared opening bytes cannot separate them on their own.

Does a matching verdict mean the file is safe?

No. A match only says the opening bytes look like the format the name claims. Safety depends on where the file came from and what it contains, which this page does not examine.

Why is a text file reported with lower confidence?

Text has no signature bytes at all. The check can only say the opening bytes decode as UTF-8 without control characters, which many text formats share with each other.

How much of my file is read?

A few kilobytes from the start, plus a slightly larger window for a ZIP container so the first entry name can be found. The rest of the file is never opened.

Project manager: Tony Hines · Content updated 1 October 2026 · Report a problem

MIME Type and File Signature Checker

Inspect file signatures controls