Steps to reproduce:

1. Configure the webhook integration between zabbix <-> mattermost: https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/media/mattermost?at=release/5.0
2. Notifications are properly sent, but there is no notification details 

Result:

Alert messages are sent, but the notification details are missing:

Expected:

I expect the same behavior of the slack integration: