Fix bug with file exists error for local

This fixes #35.

The only thing about this which I'm not 100% will work for all compressions is the file extension. We need the file extension for compressors like gzip because otherwise we get the following error:

```
BigName\BackupManager\ShellProcessing\ShellProcessFailed: gzip: .../5412f7b8f3682: unknown suffix -- ignored
```

This is because the uuid name for the working file doesn't has an extension.