Path with 36 characters is not a string uuid

Not only filenames with 36 characters are treated as string uuids, also paths like `assets/.............../js/vanilla.js` (sry for obfuscation ...).
![uuid](https://cloud.githubusercontent.com/assets/343404/3101878/6098f68e-e63d-11e3-8608-17d06a303aed.png)

Inside the `FilesModel` there was only a simple `strlen` check, now it also check the value so the false-positive should be greatly reduced :-)