Make the file storage temporal to adapt it to Heroku (files stored that do not belong to the repository are deleted after ¿30 minutes?).
Moreover, the project should have a new folder with some small size demos and a startup check to add those demo files to the database if them do not exist.
Due to the temporal behavior of Heroku storage, the better way is allowing the server to delete the files itself. If Heroku has a 30 minutes deadline, on files list request the server may delete all files with a creation timestamp older than 25 minutes (file system and database).
The client should have a new message to indicate the temporal behavior of the storage and when a file of the list is clicked it should indicate this information as well in the error message.