Steps to reproduce:

1. Import attached media type (webhook example).
2. Test imported media type (Zabbix server should be running)
3. Observe that error is present.

Result:

At this point zabbix_server.log file will become corrupted as well as it will contain broken UTF-8 sequence:

JSON retrieved from Zabbix Frontend:

{
    "string": "\ud83d\ude22"
}

At some point it becomes broken.

Expected:
Media type testing should work just like the action operations.

The same media type will work like a charm (without errors) if used in action operations. So problem is only related to media type testing.