Cannot read property 'RNFSFileTypeRegular' of undefined

react-native-fs has a RNFSManager which has some constants and other fairly low level stuff.
There is a /windows module but it is using cs and is designed for RNWcurrent and so RNWvNext and it's use of 'uwp' makes this not work.

https://github.com/itinance/react-native-fs/tree/master/windows/RNFS

Also rn-fetch-blob doesn't have windows support. And it is sorta related to file system things.
https://github.com/joltup/rn-fetch-blob