The solution turned out to be quite simple: you have to call the operator using partial() first then expand(), like this:

SalesforceBulkOperator.partial(
      salesforce_conn_id='salesforce_conn_id',
      task_id='bulk_insert',
      operation='insert',
      object_name='Attachment',
      external_id_field='Id',
      batch_size=10000,
      use_serial=False,
  ).expand_kwargs(payload)