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Check whether the file is in .jif format.

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <input type="file" name="" id="fileInput" />
  </body>

  <script>
    const fileInput = document.querySelector('input[type="file"]');
    fileInput.addEventListener("change", function (event) {
      const file = event.target.files[0];
      const reader = new FileReader();
      reader.onload = function (e) {
        const arr = new Uint8Array(e.target.result);
        // Convert to string, check JFIF signature
        const header = String.fromCharCode.apply(null, arr.subarray(6, 10));
        if (header === "JFIF") {
          console.log("File is a JFIF format.");
        } else {
          console.log("File does not contain JFIF signature.");
        }
      };
      // Read the first 10 bytes of the file
      reader.readAsArrayBuffer(file.slice(0, 10));
    });
  </script>
</html>
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