What kind of file is this?
Find out what a file really is from its contents, not its name. Catches renamed files, missing extensions and files that aren’t what they claim.
Any file, any size. Only the first and last few kilobytes are read.
Files are checked on this device. Nothing is uploaded.
When this helps
An upload form says the file type isn’t allowed even though it ends in .jpg. An attachment won’t open in anything. A download has no extension at all. A file called invoice.pdf behaves strangely. In each case the question is the same, what is this file really and the answer is in its first few bytes rather than its name.
Fixing a wrong extension
If the name is wrong, the tool offers a copy with the right extension and the file then opens normally. That only corrects the label. If you need a different format, such as a JPG for a form that refuses HEIC, convert it instead, which changes the contents.
Programs in disguise
A classic trick is a Windows program named to look like a document, such as statement.pdf.exe with extensions hidden. The tool reports programs clearly whatever they’re called. It isn’t an antivirus scanner, but a "document" that turns out to be a program is a strong reason not to open it. To check a download is exactly what its publisher released, compare its checksum.
Common questions
How can it tell what a file is without the extension?
Nearly every file format begins with a fixed signature, sometimes called a magic number, so the program opening it can confirm what it has. A PNG always starts with the same eight bytes, a PDF with %PDF, a ZIP with PK. The tool compares the start of your file with around fifty signatures.
Why would a file have the wrong extension?
Usually someone renamed it, hoping to convert it. Changing .heic to .jpg doesn’t convert anything: the contents stay HEIC and an upload form that checks the contents rejects it. Some websites also serve images in a different format from the name, so a saved photo.jpg can really be WebP.
Can it tell a Word document from a zip file?
Yes. Modern Office files, EPUB books, Java archives and Android apps are all zip files inside, so the tool looks at what the zip contains to tell them apart. Older Office files share a different container and are reported as a pre-2007 Office document.
Does it read the whole file?
No. It reads the first and last 64 kilobytes, which is where the signatures and a zip file’s index live, so even a multi-gigabyte video is checked instantly. Nothing is uploaded.
Is a file safe if the type matches?
Not necessarily. A matching type means the file is what its name says, which rules out one common trick, a program disguised as a document. It doesn’t scan for malicious content. For that, use your antivirus software.
Guides
The other tools
All of them work the same way: the file is read in your browser and never uploaded.