Steps to reproduce:
1. Open the attached project(localWWW.zip)
2. Open the Assets/localWWW.unity scene
3. Switch platform to Windows Store
4. Go to Pictures, create a folder, create a text file with any content in that folder
5. Update the script in the repro project to point to the text file you just created
6. Build for WSA Universal 10 SDK, Build type - XAML
7. Open Visual Studio solution
8. Update the Package.appxmanifest file capabilities. Add the Pictures library capability, so you can access the file created in step 4
9. Deploy on local machine (Debug)
10. Observe the exception

Expected result: WWW should read the local file
Actual result: WWW does not read the local file

Note:
In Editor works fine

Reproduced with: 5.5.1p4, 5.5.3p3, 5.6.1f1, 5.6.1p1, 2017.1.0b5
Not reproduced with: 5.5.1f1