Fixed filename which was returning random strings

cf.

```
[103] pry(main)> URI.decode('https://wulst.s3-eu-west-1.amazonaws.com/uploads/attachment/file/140/guy.rb?AWSAccessKeyId=24g24t82g4248g&Signature=Efq9pbU/sOtpm6SdDquaVzuY9/Q%3D&Expires=1418659279').gsub(/.*\/(.*?$)/, '\1').split('?').first
=> "Q=&Expires=1418659279"
```

The regex fails if the signature contains slashes. I might add a test. Just noting that not to forget about it.