Request data empty with formData - Django Rest Framework

I am trying to save strings, floats and an image via a POST request made to a Django Rest Framework backend:

# My view:

class CreatePost(APIView):
    # tried with and without parser:
    # parser_classes = [FormParser, MultiPartParser]

    def post(self, request):
        serializer = PostSerializer(data=request.data)
        if serializer.is_valid(raise_exception=True):
            serializer.save(user_id=request.user.pk)
            return Response(serializer.data, status=201)
        return Response(serializer.errors, status=400)


# My serializer:

class PostSerializer(serializers.ModelSerializer):
    class Meta:
        model = Post
        fields = ("x", "y")

And my JS frontend:

const postBody = new FormData();
postBody.append('x', 'someValue');
postBody.append('y', 'someValue');

fetch('/api/v1/post', {
    method: 'post',
    body: postBody,
    credentials: 'same-origin',
    headers: {
      'X-CSRFToken': getCookie('csrftoken'),
      'Content-Type': 'multipart/form-data'
    }
  }

However, sending as 'Content-Type': 'multipart/form-data' does not show any values in request.data in my backend. Sending as application/json works, but does not when one of the fields is an image.

How can I process both text and image using form-data? I tried using parsers (FormParser and MultiPartParser) without success.