Free, private, right here in your browser
What’s really
in that file?
Check a file’s real type from its contents, see its signature, and spot misleading extensions. Your files stay on your device.
Drop your mystery files here.
Or choose files from your device. No account. No uploads.
Up to 50 files per batch. File contents are never rendered or executed.
Use is subject to our terms. Read the privacy policy.
How this file checker works
Many formats start with a recognisable sequence of bytes—a file signature, sometimes called “magic bytes”. The checker reads a small sample from your file and compares it with known signatures. For ordinary ZIP containers it also checks the directory for Word, Excel and PowerPoint entries.
A filename can be changed without changing those bytes. That’s why a file named holiday.pdf can turn out to be MPEG-4 media.
How to identify an unknown file · What an extension mismatch means
What it can identify
Common image formats including PNG, JPEG, GIF, WebP, TIFF, HEIC and AVIF; PDF and RTF documents; ZIP and common compressed archives; supported Office containers; several media, font, database and executable formats.
Text detection is conservative. ZIP64, encrypted or unusual containers, legacy Office subtypes, metadata, Content Credentials, and file repair are beyond this browser checker’s scope. Unknown means the sampled bytes weren’t enough to identify a supported format.
A quick sniff here.
A closer look in the app.
Use the native app for broader detection, folder scanning, image metadata and embedded provenance. On Mac, you can also fix extensions and convert Outlook messages.
Your files stay yours.
Are my files uploaded?
No. Your browser reads the selected files locally. There is no file-upload endpoint, analytics script, or third-party dependency. Hosting requests for the website itself are separate from your files. See our privacy page.
Is there a file size limit?
The checker reads at most 64 KiB from the beginning of each file. For ZIP files, it also reads the final 65,557 bytes and a directory of up to 1 MiB, with at most 4,096 entries. It does not load entire large files or decompress archives. You can inspect up to 50 files per batch.
Does changing an extension convert a file?
No. Renaming changes the label, not the contents. This checker never renames or modifies your files.