Add mimetypeicon to subjectlisting

@jone Can you take a look?
I added the mimetypeicon to the subjectlisting. Per default it's disabled. You can enable it over portal_registry:

![bildschirmfoto 2013-07-16 um 13 32 37](https://f.cloud.github.com/assets/557005/804354/795c7b4e-ee0b-11e2-8a2c-b137c93e9e5d.png)

![bildschirmfoto 2013-07-16 um 13 27 07](https://f.cloud.github.com/assets/557005/804355/795d1ae0-ee0b-11e2-8909-e8959571cecb.png)