经过排查，是因为download_url 的conten-type不是video/mp4 而是video/mpeg4导致，部分手机机型出现下载invalid file type

wx.downloadFile({

url: download_url,

success(res) {

var temp = res.tempFilePath

wx.saveVideoToPhotosAlbum({

filePath: temp,

success:function(res){}

fail: function (err){}

})

}

})