Keep original file name when importing files

This pull request is rest of this discussion: https://github.com/concrete5/concrete5/pull/1468
File sanitize method is already fixed and merged in https://github.com/concrete5/concrete5/pull/1494

![file_manager](https://f.cloud.github.com/assets/514294/2201933/4d4a26ae-98f7-11e3-888f-8ab6d555173d.png)
![file_manager_1](https://f.cloud.github.com/assets/514294/2201932/4d49a5d0-98f7-11e3-88b6-63865a6b3450.png)