Do those imports!

models/activities.js

@@ -1,3 +1,5 @@
+import Attachments from './attachments';
+
 // Activities don't need a schema because they are always set from the a trusted
 // environment - the server - and there is no risk that a user change the logic
 // we use with this collection. Moreover using a schema for this collection

models/boards.js

@@ -8,6 +8,7 @@ import {
   TYPE_TEMPLATE_CONTAINER,
 } from '/config/const';
 import Users from "./users";
+import Attachments from './attachments';
 
 Boards = new Mongo.Collection('boards');
 

models/exporter.js

@@ -1,6 +1,9 @@
 import moment from 'moment';
 const Papa = require('papaparse');
 import { TAPi18n } from '/imports/i18n';
+import Attachments from './attachments';
+
+//const stringify = require('csv-stringify');
 
 // exporter maybe is broken since Gridfs introduced, add fs and path
 export class Exporter {