In the context of the local dev environment this works great, since the application is running from the project’s root folder. In prod, however, your application is running from within _build/prod/rel/..., thus why the file isn’t copied where you intended to when you pass a relative path.
Now, the uploads directory is pretty much a config concern, meaning I would personally prefer to have it in a config file rather than in a module’s function or a module’s attribute. For prod, pass the absolute path to it.
Example:
# dev.exs
config :labs, upload_path: "priv/static/uploads"
# prod.exs
config :labs, upload_path: "/var/www/labs/priv/static/uploads"
Retrieve it with Application.fetch_env!(:labs, :upload_path)
