Fix expandMapping for filenames with dots

This PR address a bug I've run into when using expand for the files config, and specifying an extension.

If `ext: 'min.js'` then `production.application.js` is mapped to `production.min.js` instead of `production.application.min.js`. In other words it scraps everything after the first dot in a filename.

![1344073006102](https://f.cloud.github.com/assets/520550/1172848/b0f19d92-212c-11e3-94d3-a77faf5343a7.gif)