"the new mimetypes should be placed in the "mimetypes" module folder."

This means that (if you want to use the mimetypes function of Iconizer module) you have to put your new mime-types icons in the "mimetypes" module folder.

You can find a set of icons form my blog here: http://thepanz.netsons.org/post/drupal-iconizer-module-d7-icons packaged with the new Admin icons.

Please set this issue to "fixed" if my reply fulfills your request.