File Signature Detector
Identify what a file really is from its magic bytes, regardless of its extension.
Fill in the form and press “Identify file” to see the result here.
How to use it
- 1Select your file, or drag and drop it onto the upload area.
- 2Adjust the options if you need something other than the defaults.
- 3Press the button. Everything happens on your device.
- 4Download the result.
About the File Signature Detector
Almost every binary format starts with a fixed byte sequence: a PDF begins with %PDF, a PNG with a specific eight-byte header, a ZIP with PK. Those bytes are evidence; the file name is only a claim.
The distinction matters for security. Any upload validation that trusts the extension or the browser-reported MIME type can be bypassed by renaming a file, which is why server-side validation has to read the leading bytes - as the upload endpoints on this site do.
Note that DOCX, XLSX, PPTX, ODT, EPUB and JAR files are all ZIP archives underneath, so they share the same signature. Telling them apart requires looking inside.
Questions people ask
Why does my .docx show as a ZIP?
Because it is one. Modern Office formats are ZIP archives containing XML. The signature check is correct; distinguishing them means inspecting the files inside the archive.
Is your file uploaded to a server?
No. This tool runs entirely in your browser using standard web APIs, so your file never leaves your device. You can confirm it by opening your browser's network panel while the tool runs, or by disconnecting from the internet after the page has loaded.
Is it free, and do I need an account?
Yes, it is free, and there is no account, no sign-up and no watermark on the result. The project is funded by optional donations.