微信支付证书签名验证是用该签名串"GET\n/v3/certificates\n{}\n{}\n\n".format(timestamp, random_str)进行验证是成功的， 然后再使用微信支付图片上传的签名验证(签名串：
"POST\n/v3/merchant/media/upload\n{}\n{}\n{'filename': 'test.png', 
'sha256': '8a631fba88f9010992c87d8098e12c5d36e9c3722daeaf33fb7dce02a2fbc083'}\n".format(timestamp, random_str))是获取了这个错误信息：
{"code":"SIGN_ERROR","detail":{"detail":{"issue":"sign not match"},"field":"signature","location":"authorization",
"sign_information":{"method":"POST","sign_message_length":76,
"truncated_sign_message":"POST\n/v3/merchant/media/upload\n1593765607\nDQgFVW2OJPACa4LEHFzIGIgWLSrfGUeo\n\n","url":"/v3/merchant/media/upload"}},
"message":"错误的签名，验签失败"}
图片上传API，请使用meta对应的JSON报文。 这个是什么意思