上传base64图片，获取图片MD5
base64图片转换
//base64转化ArrayBuffer再SparkMD5
function _base64ToArrayBufferToMD5(base64) {
const buffer = uni.base64ToArrayBuffer(base64)
var spark = new SparkMD5.ArrayBuffer();
spark.append(buffer);
var hexHash = spark.end(false);
return hexHash;
}