Validator for file-or-preserverd-as-paper needs to consider mails

Currently the [validator that checks that a document is either preserved as paper or has a digital file](https://github.com/4teamwork/opengever.core/blob/feature_public_trial/opengever/document/behaviors/metadata.py#L150) does not properly handle mails, because their file field has a different name (`message`).

This makes it impossible to disable `preserved as paper` for mails:

![bildschirmfoto 2014-07-29 um 15 44 19](https://cloud.githubusercontent.com/assets/405124/3735685/ea847138-1726-11e4-8b42-4e6b311bcc3d.png)