How to check what type a file really is
- Drop one or more files of any kind.
- See the detected file type, its MIME type and usual extension, read from the file's own signature ("magic bytes").
- If the file's extension does not match its content, you get a warning, for example a "photo.jpg" that is really a PNG or a ".pdf" that is actually a ZIP. Open the hex view to see the first bytes yourself.
Magic bytes, not extensions
File extensions are just part of the name and can be wrong, missing or deliberately misleading. Most file formats begin with a fixed signature: PNG files start with the bytes 89 50 4E 47 ("‰PNG"), PDFs with "%PDF", ZIP files with "PK", Windows programs with "MZ". The checker compares the beginning of your file against signatures for about 50 common formats, including images, audio, video, archives, documents, fonts, executables and databases, and looks inside ZIP files to tell DOCX, XLSX, PPTX, EPUB and JAR apart. Files without a signature are analysed to tell text from binary data.
This is useful when a file will not open, after downloading something with no extension, before uploading to a site that rejects the type, and as a basic safety check: an "invoice.pdf" that is actually a Windows program is a red flag. Only the first 40 KB of each file is read, locally in your browser; nothing is uploaded.
Can I fix a wrong extension?
Yes. Rename the file to the suggested extension and it should open in the right app.
Is it a virus scanner?
No. It only identifies the format; use antivirus software to check for malware.
Why is a DOCX shown as a ZIP?
Office files are ZIP archives inside. The checker looks for Word, Excel and PowerPoint folders to tell them apart.