docs: def find(file_node, dirs=ICON_DIRS, default_name=None, file_ext='.png'):
          """
          Iterating all icon dirs, try to find a file called like the node's
          extension / mime subtype / mime type (in that order).
          For instance, for an MP3 file ("audio/mpeg"), this would look for:
          "mp3.png" / "audio/mpeg.png" / "audio.png" 
          """
          names = []
          for attr_name in ('extension', 'mimetype', 'mime_supertype'):
              attr = getattr(file_node, attr_name)
              if attr:
                  names.append(attr)
          if default_name:
              names.append(default_name)
          icon_path = StaticPathFinder.find(names, dirs, file_ext)
          if icon_path:
              return StaticIconFile(file_node, icon_path)