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But in general, sandboxing means that apps can't access other apps, their data, or their files. An app has only access to its own folder (sandbox). Therefore the focus is on apps, and, while apps do work with files under the hood, you don't usually see them. Some apps do allow you to see files, but it depends on the apps.
So I have DD-WRT on the router already, but it's a really old build. I want to update the build, but everything I read says you use dd-wrt .bin firmwares to upgrade builds. All I can find for the Asus RT-AC66U is the .trx firmware which is the initial firmware file that you flash from within the stock Asus firmware to switch it over to dd-wrt.