title: Null in response which should't exist Django
body: <p>When I send request in Postman like this:
  <a href="https://i.stack.imgur.com/x6JxK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/x6JxK.png" alt="enter image description here" /></a></p>
  <p>it returns me all fields fine expect the profile_photo. profile_photo in response is null and I don't know what is the problem. It should be the new uuid photo name.</p>
  <p>Here is my model:</p>
  <pre><code>class User(AbstractBaseUser, PermissionsMixin):
      id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
      email = models.EmailField(_('email address'), unique=True)
      first_name = models.CharField(max_length=150, blank=True)
      last_name = models.CharField(max_length=150, blank=True)
      city = models.CharField(max_length=150, blank=True)
      description = models.CharField(max_length=1000, blank=True) 
      profile_photo = models.CharField(max_length=500, blank=True)
      date_joined = models.DateTimeField(default=timezone.now)
      about = models.TextField(_(
          'about'), max_length=500, blank=True)
      is_staff = models.BooleanField(default=False)
      is_active = models.BooleanField(default=True)
  
      objects = CustomAccountManager()
      object = models.Manager()
  
      USERNAME_FIELD = 'email'
      REQUIRED_FIELDS = ['first_name', 'last_name', 'city']
  
      def __str__(self):
          return self.email
  </code></pre>
  <p>This is my view:</p>
  <pre><code>class CustomUserUpdate(generics.UpdateAPIView):
      permission_classes = [IsAuthenticated]
      queryset = User.objects.all()
      serializer_class = UserUpdateSerializer
  
      def get_object(self):
          return self.request.user
  </code></pre>
  <p>This is my serializer. Here i am setting the new photo name. When I print the new name here like this: print(instance.profile_photo) it shows me the new file name fine. But in the response i get null.</p>
  <pre><code>class UserUpdateSerializer(serializers.ModelSerializer):
      profile_photo = serializers.FileField(required=False)
      class Meta:
          model = User
          fields = ('email', 'first_name', 'last_name', 'city', 'description', 'profile_photo')
  
      def __init__(self, *args, **kwargs):
          super(UserUpdateSerializer, self).__init__(*args, **kwargs)
          self.fields['email'].required = False
          self.fields['description'].required = False
  
      def update(self, instance, validated_data):
          if instance.id == self.context['request'].user.id:
              instance.email = validated_data.get('email', instance.email)
              instance.first_name = validated_data.get('first_name', instance.first_name)
              instance.last_name = validated_data.get('last_name', instance.last_name)
              instance.city = validated_data.get('city', instance.city)
              instance.description = validated_data.get('description', instance.description)
              uploaded_file = validated_data.get('profile_photo', None)
              if uploaded_file:
                  validate_extension(uploaded_file.name)
                  instance.profile_photo = handle_uploaded_file(uploaded_file)
              instance.save()
          return instance
  
  def handle_uploaded_file(file):
      extension = os.path.splitext(file.name)[1]
      new_filename = f&quot;{uuid.uuid4()}{extension}&quot;
  
      destination = open(f'static/user/images/{new_filename}', 'wb+')
      for chunk in file.chunks():
          destination.write(chunk)
  
      destination.close()
      return new_filename
  
  def validate_extension(filename):
      extension = os.path.splitext(filename)[1].replace(&quot;.&quot;, &quot;&quot;)
      if extension.lower() not in ALLOWED_IMAGE_EXTENSIONS:
          raise serializers.ValidationError(
              (f'Invalid uploaded file type: {filename}'),
              code='invalid',
          )
  </code></pre>