Base64 编码知识

https://tech.upyun.com/tech/article/715/Base64%20%E7%BC%96%E7%A0%81%E7%9F%A5%E8%AF%86%EF%BC%8C%E4%B8%80%E6%96%87%E6%89%93%E5%B0%BD%EF%BC%81.html