如何获取数组缓冲区对象的mime类型？

function getMimeType(buffer) {
  const magicNumbers = {
    '89504e47': 'image/png',
    '47494638': 'image/gif',
    'ffd8ffe0': 'image/jpeg',
    '25504446': 'application/pdf',
    '52617221': 'application/x-rar-compressed',
    '504b0304': 'application/zip',
    '4d5a9000': 'application/msword',
    'd0cf11e0': 'application/vnd.ms-powerpoint',
    '3c3f786d': 'text/html',
    '7b5c727466': 'application/msword'
  };

  const arr = new Uint8Array(buffer);
  let header = '';
  for (let i = 0; i < 4; i++) {
    header += arr[i].toString(16);
  }

  return magicNumbers[header] || 'application/octet-stream';
}

// 示例用法
const buffer = new ArrayBuffer(4);
const view = new Uint8Array(buffer);
view[0] = 0x89;
view[1] = 0x50;
view[2] = 0x4e;
view[3] = 0x47;

console.log(getMimeType(buffer)); // 输出: image/png