I haven't seen a valid ad in quite some time on mobile. All I ever see are those fake battery apps and fake virus scanners. If only Google had the balls to do this for Android... Until then, I'm going to keep rooting my phone, or ditch smartphones all together.
The only difference between a file server and a NAS is the ridiculously bad CPU and slow, clunky software it's packed with. If you really want a file server, just grab a random linux distro and install it on a PC with a lot of disks.