Upload files to AWS S3 and then attach to instance DRF

I have separate API endpoint to upload files to AWS S3 storage.

class TaskFilesUploadView(generics.GenericAPIView):
    """
    APIView for uploading task files into cloud.
    """

    parser_classes = (MultiPartParser,)
    serializer_class = TaskFileUploadSerializer

    def post(self, request, *args, **kwargs):
        file_obj = request.FILES["file"]
        file_directory_within_bucket = "task_files"
        file_path_within_bucket = os.path.join(file_directory_within_bucket, file_obj.name)
        default_storage.save(file_path_within_bucket, file_obj)
        file_url = default_storage.url(file_path_within_bucket)
        return Response(
            {
                "message": "File uploaded successfully",
                "file_url": file_url
            },
            status=status.HTTP_200_OK
        )

I have a Task model where a single instance can have multiple files.

class TaskV1(models.Model):
    """
    Task table model.
    """
    ...

class TaskFileV1(models.Model):
    """
    Since task can have several files in one task,
    we need this table with FK to TaskV1.
    """
    task = models.ForeignKey(TaskV1, on_delete=models.CASCADE, related_name="files")
    file = models.FileField()

During task creation client attaches files and then pushes "save task" button, so i need to pass recieved file urls into json.

{
  ...
  "files": [
    {
      "file": "string"
    }
  ]
}

But i got HTTP 400 BAD REQUEST with following error:
The submitted data was not a file. Check the encoding type on the form.
How should it be implemented?