[Plugin: WP-DownloadManager] how to add icons for custom file types

in case anybody else wonders about this: it is very easy to add icons for additional file types to the plugin.
just create a 16×16 pixel GIF file, name it [fileextension].gif (i.e. zip.gif for zip-files) and upload it to /wp-content/plugins/wp-downloadmanager/images/ext
the plugin will do the rest.