Javascript – unable to send multiple files to backend React/ NestJS / BAD REQUEST 400

I am trying to make this feature when user can upload both CV and his image.

However whenever I try to send both at the same time to backend I get bad Request error 400. I tried to change things but I just cant seem to figure that out why it goes to 400 , when I upload image , or cv while commenting out first one , or the latter one , and when done separately it works fine.

here is my backend code:

  @Post('signup')
  @UseInterceptors(FileInterceptor('cv'), FileInterceptor('avatarImage'))
  async signUp(
    @Body() signupDto: SignupDto,
    @UploadedFile() cv: Express.Multer.File,
    @UploadedFile() avatarImage: Express.Multer.File,
  ): Promise<any> {
    if (!cv) {
      throw new Error('CV file is required.');
    }
    signupDto.cvFileName = cv.originalname;
    signupDto.cvFileBuffer = cv.buffer;

    signupDto.cv = await this.fileUploadService.uploadFileCv(
      signupDto.cvFileName,
      signupDto.cvFileBuffer,
    );

    if (!avatarImage) {
      throw new Error('Avatar Image is required');
    }

    signupDto.avatarImageName = avatarImage.originalname;
    signupDto.avatarImageFileBuffer = avatarImage.buffer;

    signupDto.avatarImage = await this.fileUploadService.uploadImage(
      signupDto.avatarImageName,
      signupDto.avatarImageFileBuffer,
    );
    
    return this.userService.createUser(signupDto);
}

I can send them separately , but I cant send them at the same time.
in addition I provide snippet of fetching logic.

  const handleRegistration = async (event: FormEvent<HTMLFormElement>) => {
    event.preventDefault();

    const formData = new FormData();
    if (selectedCvFile.length > 0) {
      formData.append("cv", selectedCvFile[0] || "");
    }
    if (selectedImagesForAvatar.length > 0) {
      formData.append("avatarImage", selectedImagesForAvatar[0] || "");
    }
 
    formData.append("username", formState.username || "");
    formData.append("firstName", formState.firstName || "");
    formData.append("lastName", formState.lastName || "");
    formData.append("password", formState.password || "");
    formData.append("email", formState.email || "");
    formData.append("phoneNumber", formState.phoneNumber || "");
    formData.append("address", formState.address || "");
    formData.append("city", formState.city || "");
    formData.append("role", formState.role || ""); 
    dispatch({ type: "CLEAR_ALERTS" });
    try {
      const response = await fetch("http://localhost:3000/auth/signup", {
        method: "POST",
        body: formData,
      });

      const data = await response.json();

      if (response.ok) {
        dispatch({
          type: "SHOW_SUCCESS",
          payload: `You have registered correctly !`,
        });
        console.log(data);
        localStorage.setItem("token", data.accessToken);

      } else {
        dispatch({ type: "SHOW_ERROR", payload: response.statusText });
        console.error("Registration failed:", response.statusText, response);
        // Handle failed registration, e.g., show an error message
      }
    } catch (error) {
      console.error("Error during registration:", error);
      // Handle any other errors that may occur during registration
    }
  };

If you have idea what I can do to make it work , please do so : )

Thanks