Steps to reproduce:

1. Import Mattermost Media Type for Zabbix Server 6.4 from official git repository (See attached .yaml file)
2. Configure Media type to send alerts to Mattermost
3. See Zabbix Web interface for error (attached image)
4. See logs at debug level 4 (attached image)

Result:
Mattermost notification fails with message: SyntaxError: invalid json (at offset 2)
Expected:
It is expected to send the alerts successfully since the configuration worked correctly on Zabbix Server 4.x. After reimporting the template for the new version it still throws an error.