接口:

https://api.weixin.qq.com/cgi-bin/message/mass/preview?access_token=ACCESS_TOKEN

入参:

https://api.weixin.qq.com/cgi-bin/message/mass/preview?access_token=xxx,{"touser":"xxx","mpnews":{"media_id":"s7zzn6zaRem-_8MUzSxs6MvUdvl0SnhBttiplJsheUfI2EHA5vu_aWPFXgXfFivE"},"msgtype":"mpnews"} 

出参:

{"errcode":40229,"errmsg":"empty media cover, please check the media rid: 6731c929-5ce9e137-2c4e27e1"}