Hi Ivan,
To store files in Mendix, you need to create an entity in your domain model which is a specialization of the System.FileDocument entity and grant the required access to the module roles involved.
To manage these files (even when you want just 1 record), I would suggest to create an overview page showing "all records" of that entity and a detail page to edit a single record.
Running the app, you should then be able to upload and download files.
Hopefully this helps!
Regards,
Johan