grunt rename function not working

Question

I am using save2png to provide fallback pngs to svg4everybody. The svgs are created and I can change input and output directory. But svg4everybody requires that the pngs are saved with a filename like: "spritesheetname.svg.id.jpg"

At first, I tried some simple things, but I can't even figure out how to change the filename in the first place.

Here is my code:

module.exports = function(grunt){

    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),
        svg2png: {

            all: {
                files: [
                    // rasterize all SVG files in "img" and its subdirectories to "img/png"
                    {   cwd: '../img/icons/',
                        src: ['**/*.svg'],
                        dest: '../symbol/svg/',
                        rename: function(dest, src) {
                            var filename  = src.substring(src.lastIndexOf('/'), src.length);
                            return dest + 'prefix' + filename + '.png';
                        }
                    }
                ]
            }
        }
    });

    grunt.registerTask('default', []);
    grunt.loadNpmTasks('grunt-svg2png');

};

The pngs are created, but the filename is always the same as the original svg.

I stumbled upon this question, Grunt rename not working, but it was no real help. Flatten and expand just changed the directory structure, but had no influence on the filename.