实现方法

/**
 * 从base64编码的图片中获取扩展名
 * @param {String} base64 
 * @returns 
 */
function getExtensionFromBase64(base64) {
  let re = new RegExp('data:image/(?<ext>.*?);base64,.*')
  let res = re.exec(base64)
  if (res) {
    return res.groups.ext
  }
}

测试

let base64 = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEU'
let ext = getExtensionFromBase64(base64)
console.log(ext)
// png