.JXL file signature
A JPEG XL image file begins with the bytes FF 0A at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless JXL file is still recognisable.
Known signatures
| Offset | Hex | ASCII |
|---|---|---|
| 0 | FF 0ABare codestream. | |
| 0 | 00 00 00 0C 4A 58 4C 20 0D 0A 87 0AISO container form. | ....JXL .... |
MIME type: image/jxl · usual extensions: .jxl
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.jxl
file file.jxlWindows (PowerShell):
Format-Hex -Path file.jxl -Count 32If the bytes do not match
A file named .jxl whose first bytes are something else is not a JXL file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the JXL guide explains what a genuine file should contain.