Mail preview tab: fixed bug in mimetype lookup for attachments with a wrong mimetype

When a mail contains attachment with a mimetype, wich is not registered in the mimetypes_registry, the mimetype lookup fails with the following error message:
```
/home/zope/eggs/Zope2-2.13.18-py2.7.egg/Products/PageTemplates/Expressions.py:117→ render
/home/zope/eggs/plone.memoize-1.1.1-py2.7.egg/plone/memoize/instance.py:51→ memogetter
/home/zope/clients/plone01-ska-arch-dev/src/opengever.core/opengever/mail/browser/mail.py:52→ attachments
/home/zope/clients/plone01-ska-arch-dev/src/opengever.core/opengever/mail/browser/mail.py:35→ lookup_mimetype_registry
```

This PR fix this bug. @lukasgraf please take a look ...