There are two options to have a permanent link to an uploaded file: REST-API and a custom Spring controller.
You can use the first option if you require authentication from an external source. REST-API file downloading is described here: https://doc.cuba-platform.com/manual-6.3/rest_api_v2_ex_file_download.html
If you want to show images without any authentication then you have to create a custom Spring controller. At the moment it is not that simple task and requires a lot of manual work with internal CUBA API. I’ve created a small project that shows how to create such a custom download controller: https://github.com/cuba-labs/public-file-download There I use special PublicFile entity with a reference to FileDescriptor and export file content using special PublicFileDownloadController.
We are planning to improve API for FileStorage in one of the next versions of the platform where we will introduce simple API for obtaining a file stream from a middleware.