Invalid according to Policy: Policy expired.

小程序直传OSS时报错：

Object {cookies: Array[0], data: "
cookies: Array[0]
data: "AccessDenied Invalid according to Policy: Policy expired.5FE30C24CB3DB13837E8A03Fhrncardvideo.oss-cn-beijing.aliyuncs.com"
errMsg: "uploadFile:ok"
header: Object
statusCode: 403
__proto__: Object

服务端签名代码：

public static string GetEncodedPolicy(int maxSize,string startWith)
        {
            if (maxSize==0)
            {
                maxSize = 20 * 1024 * 1024;//20M
            }
            // 创建OssClient实例。
            var client = new OssClient(endpoint, accessKeyId, accessKeySecret);
            DateTime now = DateTime.Now;
            DateTime ex = now.AddSeconds(15);
            PolicyConditions policyConds = new PolicyConditions();
            //policyConds.AddConditionItem("content-length-range", 0L, 1048576000L);
            policyConds.AddConditionItem("content-length-range", 0L, maxSize);
            policyConds.AddConditionItem(MatchMode.StartWith, "key", startWith);
            string postPolicy = client.GeneratePostPolicy(ex, policyConds);
            byte[] binaryData = Encoding.Default.GetBytes(postPolicy);
            string encodedPolicy = Convert.ToBase64String(binaryData);
            return encodedPolicy;
        }