Heroku file system is ephemeral, meaning Dyno applications can write on the file system but changes are discarded when the Dyno restarts.
It is also important to remember that Heroku Dynos restart (at least) every 24 hrs (see cycling), hence deleting all local filesystem changes.
Applications which need to persist data should rely on an external service like a database or a remote storage.
For applications that need to store files there are few remote storage cloud services offering a free tier, some of them limiting the size and/or the features available.
They all provide a secure (tokens, credentials) access via APIs and are suitable for both projects under development or in production (see details of each provider):