refactor: Upload file to storage, and then pass attachmentId to collections.import

This avoids having large file uploads going directly to the server and allows us to fetch it async into a worker process

server/models/Attachment.js

@@ -1,7 +1,7 @@
 // @flow
 import path from "path";
 import { DataTypes, sequelize } from "../sequelize";
-import { deleteFromS3 } from "../utils/s3";
+import { deleteFromS3, getFileByKey } from "../utils/s3";
 
 const Attachment = sequelize.define(
   "attachment",
@@ -47,6 +47,9 @@ const Attachment = sequelize.define(
       isPrivate: function () {
         return this.acl === "private";
       },
+      buffer: function () {
+        return getFileByKey(this.key);
+      },
     },
   }
 );