I would like to add a upload/download folder that can be used by signed in users (I implemented it with guardian) but files are not associated with any of them; file dump.
I would like to limit the type of files to for example .txt or .json if possible
Phoenix is a web framework, not a webserver such as NGINX or Apache.
If you want to show your user something you have to write it.
You can use File.ls/1 to list the contents of a folder,
then you can show that list and have the user pick one,
next you can use send_download/3 to send them the file.