[BUGFIX] delete temp files in format manager

to delete existing temp files run in your temp folder (normally `/tmp` or `/private/var/tmp`):
``` bash
# run this command with caution
sudo find . -type f -name media_\* -exec rm {} \;
```

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | none
| Fixed tickets | #106
| BC Breaks     | none
| Doc           |  none

@alexander-schranz use http://symfony.com/doc/2.3/components/filesystem.html to delete files! service id `filesystem` ...