How to add an additional config dir in DUB under source?

[tastyminerals]

I have the following project structure:

source/
   media/
     icon.png
   config/
     conf.toml

In order to access "icon.png" without explicitly providing the 
path I added in dub.json

"stringImportPaths": [
     "source/media",
     "source/config"
]

It works for "icon.png" but doesn't work for "conf.toml". The 
"icon.png" can be accessed and the code below works:

     addEntry(new EmbeddedPng!("quit.png")

but std.file: readText refuses to see "conf.toml":

     readText("conf.toml");

file.d(371): conf.toml: No such file or directory

I wonder why and what am I doing wrong?