We will be creating 3 new file uploads fields within the Referral Model, namely FollowUpLetterFileName, OutReachLetterFileName, ReferralFileName.
All these fields will point to the actual files stored within a Mongo's GridFSBucket instance (by filename).
You should make POST, GET, and DELETE routes to upload, retrieve, and delete each of these files into the database. If there is already a file saved, it should get replaced by the new file that is being POSTed
For a nice intro into how to use GridFS (Mongo's own file storage solution for >=16MB file size), here is a nice tutorial
FollowUpLetterFileName, OutReachLetterFileName, ReferralFileName.GridFSBucketinstance (by filename).