search mimetypesregistry before using the iconmap

@maethu I added a fix for the iconproblem. We now search the mimetype-registry first and only if we don't find a icon we use the icon map. Since the contenttype prefix alone is also used to determine the icon we have a problem that for instance a psd file would just be displayed like any other image which isn't good because it requires a special programm to be viewed(at least on windows). With a special icon we can address this issue and the best way to customize mimetype-icons is the mimetype-registry