Nas OS with App

[j.a.cole]

TLDR: Looking to build a NAS, Want an OS that also has app support for mobile upload and download

So I currently have a WD my cloud EX4100 and it has served me well for many many years now. However with the new push by WD for My cloud OS 5 after Updating my firmware I have lost a very key feature. Being able to upload files from my phone.
The New OS 5 mobile app only allows for images or videos to be uploaded from the phone to the NAS remotely. But it doesn't allow PDF's!? or any other file type, no word documents, no program files nothing. For me my NAS is a key productivity tool. I couldn't care less about backing up pictures from my phone. 
I'm planning on moving away from the proprietary boxes like WD, synology, ECT and looking to custom build, So i can build something with a bit more power for Plex encoding when I'm not using it for work related stuff..
I'm pretty comfortable with the hardware side of things. But I need an OS or OS with an add on that supports a decent remote access app, It needs to support both android and ios as my phone is android, But I also have a work provided iPad. If there's not and option for both then Android support is the priority. I was looking at Truenas currently however i dont know much about app support, and at the minimum i cant find an official app on the play store.
I already have a Static IP from my ISP for other uses like my VPN.