How to use the File Type Checker
- Choose a file.
- Read the detected type and MIME type.
- Check the warning if the extension doesn't match.
What does this tool do?
Most file formats start with a recognizable signature, called a “magic number” — PDFs begin with %PDF and PNGs with a fixed 8-byte header. The checker reads only the first few kilobytes and compares them against dozens of known signatures.
Why use it?
- Open files with no extension.
- Spot programs disguised as documents.
- Debug upload errors caused by wrong file types.
Example
A file called “invoice.pdf” that starts with “MZ” is actually a Windows program — a classic trick in phishing emails.
Privacy
Your files are processed entirely in your browser using built-in web technologies. They are not uploaded to our servers, and closing the page clears them from memory. There's no account to create and nothing to install.
Frequently asked questions
Are my files uploaded?
No. Files are read and processed in your browser, and results are saved straight back to your device.
Can a file's extension be wrong?
Yes. Renaming a file doesn't change its contents, so a file called photo.jpg can really be a PDF, an archive or a program. That's why checking the signature is more reliable.
Last reviewed by the M2Toolkit team.