public static final String CREATE_DT_MUTATION = """
            mutation($dt: CreateDTInput,$file: Upload!) {
                    createDT(dt: $dt,file: $file) {
                        id
                        dtName
                      }
                }
            """;

     and my Test method is like this:

void testBase() throws Exception {
          MockMultipartFile mockFile = new
                MockMultipartFile(
                "file",
                TR181_DT_XML,
                "application/xml",
                getResourceAsString(TR181_DT_XML).getBytes());
        String base64EncodedFile = Base64.getEncoder().encodeToString(mockFile.getBytes());
        
         GraphQlTester.Response response = graphQlTester.document(CREATE_DT_MUTATION)
                .variable("deviceType", buildBaseDT())
                .variable("file", base64EncodedFile)
                .execute();

 java.lang.IllegalStateException: Timeout on blocking read for 5000000000 NANOSECONDS
	at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:129)
	at reactor.core.publisher.Mono.block(Mono.java:1807)
	at org.springframework.test.web.reactive.server.DefaultWebTestClient$DefaultRequestBodyUriSpec.exchange(DefaultWebTestClient.java:369)
	at org.springframework.graphql.test.tester.WebTestClientTransport.execute(WebTestClientTransport.java:60)
	at org.springframework.graphql.test.tester.DefaultGraphQlTester$DefaultRequest.execute(DefaultGraphQlTester.java:179)