File Type Checker
Drop a file and read its magic bytes to find its true type, then compare that against what the extension claims. Nothing leaves your browser.
Drop a file here
or choose one – nothing leaves your browser
- File name
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- Extension
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- Detected type
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- First bytes
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What this does
Drop a file in and this tool reads its opening bytes – the file’s “magic number” – to work out what it actually is, then checks that against the extension in its name. It’s a quick way to detect a file’s real format online: to catch a .jpg that is really a PNG, spot a renamed file that won’t open, or simply confirm what a mystery file with no extension truly is.
The file never leaves your device. Only its first 32 bytes are read, in your browser, and nothing is uploaded, stored, or sent anywhere.
Why you’d want to check a file’s type
A file’s extension – the .pdf or .pngon the end of its name – is just a label. Anyone can rename a file, and the label doesn’t have to match what’s inside. Most of the time that’s harmless, but now and then it matters:
- A download won’t open, and you want to know whether the file is broken or simply misnamed.
- Someone sent you a file with a generic or missing extension and you need to identify it before you can use it.
- You’re about to open a file from a source you don’t fully trust, and you’d like to see what it really is first.
- An export or conversion has left you with a file whose extension doesn’t look right, and you want to confirm the true format before passing it on.
How it works
Most file formats begin with a short, fixed sequence of bytes called afile signature or magic number. A PNG image always starts with the same eight bytes; a PDF always begins %PDF; a ZIP archive starts PK. These signatures sit at the very start of the file and don’t depend on the name, so reading them tells you what a file genuinely is rather than what it claims to be.
This tool reads just the opening bytes, compares them against a table of known signatures, and then looks at the extension on the filename. If the two agree, you get a clean match. If the contents say one thing and the name says another, you get a mismatch, with the format the file actually appears to be. All of it happens locally – the comparison is done by a small piece of code running in your browser, not on a server.
What it can identify
The tool recognises the common formats that carry a reliable signature, including:
- Images – PNG, JPEG, GIF, BMP, TIFF, WebP, HEIC/HEIF, ICO, and Photoshop (PSD) files.
- Documents – PDF, modern Office files (DOCX, XLSX, PPTX), legacy Office files (DOC, XLS, PPT), and PostScript.
- Archives – ZIP, GZIP, BZIP2, 7-Zip, RAR, and ZIP-based formats such as EPUB, JAR, APK, and OpenDocument.
- Audio and video – MP3, FLAC, OGG/Opus, WAV, AVI, and MP4/QuickTime family files.
- Fonts and other – WOFF and WOFF2 fonts, and WebAssembly modules.
Plain-text formats such as .txt, .csv,.html, and .json have no signature to read, so they can’t be verified this way – there’s more on that below.
Common questions
How do I check what type a file really is?
Drop the file onto the box above, or choose it from your device. The tool reads its opening bytes and tells you the format it detects, alongside the extension on the name, so you can see at a glance whether they agree. You don’t need to install anything or upload the file.
Can I trust a file’s extension?
Not on its own. The extension is only a name, and it can be wrong – through an honest mistake, a botched rename, or occasionally on purpose. The contents are the reliable guide, which is exactly what this tool reads.
Is my file uploaded?
No. Only the first bytes are read locally to identify the format. The file itself is never sent anywhere, and nothing is stored. When you close the tab, it’s gone.
What is a magic number or file signature?
It’s a short, fixed sequence of bytes at the very start of a file that identifies its format – the same way the first few notes of a song can tell you which song it is. Because it lives inside the file rather than in its name, it doesn’t change when a file is renamed.
Which formats can it detect?
Common image, document, audio, video, archive, and font formats have distinctive signatures and are the most reliable to detect – see the list above. Plain text formats such as .txt,.csv, or .json have no signature, so they can’t be verified this way.
Why does my .docx show up as a ZIP?
Modern Office files (.docx, .xlsx,.pptx) are really ZIP archives inside, so they share the ZIP signature. The tool treats those extensions as a legitimate match rather than flagging them as wrong.
Does it work offline?
Once the page has loaded, the checking itself runs entirely in your browser, so it keeps working even with no connection. Nothing about the file is sent over the network at any point.