wx.cloud.getTempFileURL方法换取到的地址，为啥返回的maxAge只有86400。
errMsg: "cloud.getTempFileURL:ok"
fileList: Array(1)
0:
errMsg: "ok"
fileID: "cloud://dev-6g26owf1de0faf3e.6465-dev-6g26owf1de0faf3e-1365292025/my-photo0.png"
maxAge: 86400
status: 0
tempFileURL: "https://6465-dev-6g26owf1de0faf3e-1365292025.tcb.qcloud.la/my-photo0.png?sig
86400
可以设置成公有读