Replace everything before and expanded_dir to get file name

I had an issue in Windows where ```src_path``` contained the full path to the source file, resulting in an incorrect ```file_name```.
eg
```src_path``` was  ```C:/xampp/htdocs/myproject/bower_components/jquery/dist/jquery.js```
```file_name``` was ```C:/xampp/htdocs/myproject//dist/jquery.js```

Replacing everything before ```expanded_dir``` fixed the issue...... not sure if that's the best place to do it