Extract attachments view: fixed icon display in attachment listing.

Since the change to the new theme (`plonetheme.teamraum`), the type column in attachments has been broken.  This PR solves this issue.

![bildschirmfoto 2014-11-12 um 14 45 19](https://cloud.githubusercontent.com/assets/485755/5011126/d5eddc3a-6a7a-11e4-984a-5b0d60844a4f.png)

@deiferni please have a look
please use new-style string formatting, i.e. `.format` instead of `%`