API Call -- changing request header "Content-Type" not working? (EDIT: Request header "Content-Type" being appended with "; charset=utf-8")

I'm trying to change the value of the request header "Content-Type", but I am getting the impression that I cannot override this in FlutterFlow.

Prior to this, I tried making an API call via Postman, and that worked (since I am able to override "Content-Type" in there).

So here's how the request headers look like:

...and here's the response:

Here's the full raw response body:

{"error":{"message":"The header value is invalid. For the HTTP method ‘PATCH’, use: application/merge-patch+json.","target":"{content-type}"}}

This is a response from the target system saying that the header value for "content-type" should be "application/merge-patch+json" for the call to be processed. It seems it's receiving a different value.

So how can I override the header value for "content-type" in the outbound request from FlutterFlow?

EDIT: It seems that FlutterFlow is appending "; charset=utf-8" to "Content-Type", which is causing the receiving system to return status 400: Bad Request.