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What Is This File?

Drop a file you cannot open. We read its signature in your browser and tell you what it is.

Your files stay safe — processed entirely in your browser. Nothing is uploaded.

Drop any file here

or click to select — any format, any size

The file is read in your browser and never uploaded.

Why the extension is not the answer

A file's extension is just the end of its name. Anyone can change it, email systems strip it, and downloads routinely arrive with the wrong one. That is why a file that "should" open sometimes will not, and why an unfamiliar extension tells you so little.

Almost every binary format instead starts with a fixed sequence of bytes — a signature. A PNG always begins 89 50 4E 47. A PDF always begins %PDF-. A winmail.dat always begins 78 9F 3E 22. Those bytes are part of the file itself, so they survive renaming, and they are what this tool reads.

It currently knows 158 formats, and reads the first few kilobytes of your file to check them. When the signature is ambiguous — every Office document, EPUB, Android package and Apple Pages file is a ZIP archive underneath — it looks inside the container for the entries that tell them apart.

The file identifier after a file with no extension is dropped: the detected format, the evidence for it, and a button through to the tool that opens it.
Screenshot Identified from the contents, not the name, with the evidence shown.

What the confidence labels mean

Identified from its signature
The file's own bytes matched a known format. This is reliable regardless of what the file is called.
Identified from its contents
The wrapper was recognised and the specific format was determined by what is inside it — for example a ZIP that contains word/document.xml is a Word document.
Identified from its structure
Text formats have no signature, so these are recognised by what they contain — a vCard opens with BEGIN:VCARD, an email has RFC 822 headers.
Guessed from the file name
Nothing in the file itself matched, so this is the extension's claim and nothing more. Treat it as a hint.

Nothing is uploaded

Identifying a file you cannot open should not require handing it to a stranger — the whole reason you do not know what it is might be that it came from somewhere you do not trust. Detection runs in your browser using File.slice(), so only the handful of kilobytes needed for the signature is ever read, and none of it leaves the tab.

You can confirm that: open your browser's developer tools, switch to the Network tab, and drop a file. The page loads, and then nothing.

Formats with a guide of their own

Of the 158 formats the identifier recognises, the ones people most often receive and cannot open have a page each: why it will not open, and how to get at the contents. The result panel links to these too, but if you already suspect what the file is, go straight there.

All format guides →

Frequently Asked Questions

How does it identify a file without the extension?
Most binary formats begin with a fixed sequence of bytes called a signature — a PNG always starts with 89 50 4E 47. Those bytes are part of the file, so they survive renaming. The tool reads the first few kilobytes and matches them against the formats it knows.
Is my file uploaded?
No. It is read in your browser with File.slice(), so only the few kilobytes needed for the signature are touched and nothing leaves the tab. A multi-gigabyte file is identified just as fast as a small one.
Why does it sometimes say 'guessed from the file name'?
Because nothing inside the file matched. Text formats and some proprietary ones have no signature at all, so the extension is the only clue left — and the tool says so rather than pretending to be sure.
It says my Word document is a ZIP archive. Is that wrong?
Modern Office files really are ZIP archives of XML. The tool looks inside the container for entries like word/document.xml to name the specific format, and will tell you when it could only identify the wrapper.
What if the format is not recognised?
You get the first bytes in hex, which is usually enough to search for. It may also be an encrypted file, a proprietary format, or a fragment of a larger download.
Can it open the file too?
If it is one of the formats this site handles — winmail.dat, .msg, .eml, .vcf, HEIC or WebP — a button appears that takes you straight to the right tool. Otherwise it names the applications that open it.
How many formats does it know?
Around 150 at the moment, covering email, images, video, audio, documents, archives, fonts, databases and design files. The list grows based on what people actually bring to it.

Related articles

How to Show and Change File Extensions on Windows and Mac

Windows and macOS hide file extensions by default, which is why a file 'with no extension' often has one you cannot see. How to turn them on, how to change one safely, and what changing it does not do.