What Is This File?
Runs in your browser — your files are never uploaded.
Every other tool here asks you to name the format first, which is the one thing you cannot do when a file simply will not open. This one asks for nothing. Drop the file and it reads the signature out of the bytes themselves — the few identifying characters almost every format writes at the very start — then tells you what it is and which tools will take it. Nothing is uploaded: the bytes are read on your own device.
How to use What Is This File?
- 1
Drop the file onto the box, or click to choose one. Any format, any size.
- 2
Read what it actually is — the format, its MIME type, the file size, and the pixel dimensions where the header carries them.
- 3
Pick one of the suggested tools if you want to convert it. The file travels with you, so there is no need to choose it again.
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Frequently asked questions
Why does my file say .jpg but nothing will open it?+
Because the extension is only a label, and anything can rename a file. Phones export HEIC named .jpg, messaging apps hand over WebP named .jpeg, and downloads arrive as .txt that are really PDFs. The program opening it reads the contents, finds something it does not recognise, and refuses. Renaming it again will not help — you need to know what it truly is, which is what this page reads off the bytes.
How can it tell without opening the file properly?+
Almost every binary format begins with a short fixed sequence called a magic number. PNG starts with the bytes 89 50 4E 47, PDF with %PDF-, ZIP with PK. Reading the first few bytes is enough to name the format with certainty, and it takes no longer for a four-gigabyte video than for a thumbnail.
Is my file uploaded to identify it?+
No. The bytes are read inside your browser and nothing is sent anywhere. Files under 16 MB are read in full so that text formats like JSON can be checked properly; above that only the first 64 KB is read, which is all any binary format needs.
Why does it say the format came from the content rather than the signature?+
Text formats have no magic number. JSON, CSV, XML and Markdown are all just characters, so they are recognised by their shape instead — whether the content parses as JSON, whether every line has the same number of commas. That is a good reading rather than a certainty, and the page says which one it is rather than overstating it.
It could not identify my file. What now?+
Either the format is genuinely obscure, or the file is damaged. A download that stopped early keeps its header and is still identifiable; one that was corrupted often loses exactly the first bytes that name it. If the size looks far smaller than it should be, the download is the likely culprit.