Grunt: How to build the files object dynamically

I must be missing something very simple here. I'm trying to write a function task that deals with files. The Grunt API docs mention that you can [Build the files object dynamically], but for some reason I can't get this to work. A simplified version of my Gruntfile.js file looks like this:

module.exports = function(grunt) {
    grunt.initConfig({
        proj: {
            build: {
                files: [{
                    expand: true,
                    cwd: 'src',
                    src: ['**/*.js'],
                    dest: 'dist'
                }]
            }
        }
    });

    grunt.registerTask('proj', function(){
        var files = grunt.config('proj.build.files');
        console.log(files);
    });
};

I expect the log to show a list of file mappings from the src directory to the dist directory. What actually gets logged is the object proj.build.files from the config, like this:

Running "proj:build" task
[ { expand: true, cwd: 'src', src: [ '**/*.js' ], dest: 'dist' } ]

Done, without errors.

The API docs only talk about this type of configuration in terms of other tasks. I tried looking through the uglify task to see how the file mappings are retrieved, but I couldn't figure it out.