Spring Controller testing

I am trying to test a post method called saveProduct in my springboot application. It takes params like product, bindingResult, model, multipartFile, redirectAttributes. The product has an attribute photo.

The issue is I am getting an http status is 400 when it is supposed to be 200. I looked at the several online documentation on how to test post api but I still cant figure it out. This is one of the documentation I was looking at on how to test a post multipart uploading springboot application. https://spring.io/guides/gs/uploading-files/

Controller class (method)

@PostMapping("/showNewProductForm")
    public String addAProduct(@Valid @ModelAttribute("saved") Product product,
                              BindingResult bindingResult,
                              Model model,
                              @RequestParam("image") MultipartFile file,
                              RedirectAttributes redirectAttributes) {
        return productService
                .saveProduct(product, bindingResult, model, file, redirectAttributes);
    }

Service class (method)

public String saveProduct(Product product,
                              BindingResult bindingResult,
                              Model model,
                              MultipartFile file,
                              RedirectAttributes redirectAttributes) {
        Optional<Product> productByName = productRepository.findProductByName(product.getName());

        if (bindingResult.hasErrors()) {
            return "add-new-product"; // For binding error, you always return to the form instead of the get API
        } else if (productByName.isPresent()) {
            redirectAttributes.addFlashAttribute("message",
                    product.getName() + " already exists, please enter a different name");
            return "redirect:/showNewProductForm";
        }
        boolean checkImageUploadStatus = addingImageLogic(product, file);
        if (!checkImageUploadStatus) {
            redirectAttributes.addFlashAttribute("message", "Please enter an image for " + product.getName());
            return "redirect:/showNewProductForm";
        }
        productRepository.save(product);
        model.addAttribute("saved", product);
        redirectAttributes.addFlashAttribute("message",
                "You successfully uploaded " + product.getName() + "!");
        return "redirect:/showNewProductForm";
    }

Test class

@WebMvcTest(ProductController.class)
class ProductControllerTest {
    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private ProductService productService;


    @Test
    void addAProduct() throws Exception {
        /*
        *  Product parameters are
        *  product (name, category, description, photo)
        */
        Product product = new Product();
        product.setName("Test Run");
        product.setCategory("Dairy");
        product.setDescription("Lorem 24");
        Model model = mock(Model.class);
        BindingResult bindingResult = mock(BindingResult.class);
        RedirectAttributes redirectAttributes = mock(RedirectAttributes.class);

        // When
        MockMultipartFile mockMultipartFile = new MockMultipartFile(
                "image",
                "green-bean.jpg",
                "image/png, image/jpg, image/jpeg",
                "random img".getBytes()
        );
        when(productService
                .saveProduct(product, bindingResult, model, mockMultipartFile, redirectAttributes))
                .thenReturn(String.valueOf(product));

        // then

        this.mockMvc
                .perform(multipart("/showNewProductForm")
                        .file(mockMultipartFile))
                .andExpect(status().isOk())
                .andExpect(jsonPath("$.name", is("Test Run")));

        then(this.productService)
                .should()
                .saveProduct(product, bindingResult, model, mockMultipartFile, redirectAttributes);
    }

}

The error message

org.thymeleaf.exceptions.TemplateInputException: Error resolving template [showNewProductForm], template might not exist or might not be accessible by any of the configured Template Resolvers

Structure