migration: extract files from old storage and load into new storage

models/avatars_old.js

@@ -0,0 +1,29 @@
+AvatarsOld = new FS.Collection('avatars', {
+  stores: [new FS.Store.GridFS('avatars')],
+  filter: {
+    maxSize: 72000,
+    allow: {
+      contentTypes: ['image/*'],
+    },
+  },
+});
+
+function isOwner(userId, file) {
+  return userId && userId === file.userId;
+}
+
+AvatarsOld.allow({
+  insert: isOwner,
+  update: isOwner,
+  remove: isOwner,
+  download() {
+    return true;
+  },
+  fetch: ['userId'],
+});
+
+AvatarsOld.files.before.insert((userId, doc) => {
+  doc.userId = userId;
+});
+
+export default AvatarsOld;