Making dataTypes of existing field as strict but allowing accepting new fields dynamically


I am looking for a way to be able to specify in my template that all existing fields should be strictly checked for their data types specified in this template. But also, the new unknown fields should get accepted with ES default detection method.
As I searched for a possible way, I tried specifying "dynamic":"strict" in individual fields mapping but not at the top level so that I can still accept new fields but discard any doc that contains a known field with un-intended dataType.
But this doesn't work.

Does anyone know how this can be achieved?