WireMock

Start a wiremock server, port 8089 (I was using spring boot annotations but guess the underlying is the same?),  I had post in stubFor originally but changed to any(urlMatching...) (see below):

val responseBody = """
            {
                "attributes_import_id": 2012
            }
        """.trimIndent()

stubFor(
            any(urlMatching("/api/products/attributes/imports"))
                .willReturn(
                    aResponse()
                        .withStatus(201)
                        .withBody(responseBody)
                )
        )

Whilst the wiremock server is running, just fire a postman request at:

POST <http://localhost:8089/api/products/attributes/imports>

set the body in postman as form-data (I selected a file with some content)
The use the headers i posted above, and you should see the error?
Hope that helps? Thanks