Form the URI for a music service element.

        :param item_content: The content dict of the item
        :type item_content: dict
        :param item_class: The class of the item
        :type item_class: Sub-class of
            :py:class:`soco.data_structures.MusicServiceItem`

def form_uri(item_content, item_class):
        """Form the URI for a music service element.

        :param item_content: The content dict of the item
        :type item_content: dict
        :param item_class: The class of the item
        :type item_class: Sub-class of
            :py:class:`soco.data_structures.MusicServiceItem`
        """
        extension = None
        if 'mime_type' in item_content:
            extension = MIME_TYPE_TO_EXTENSION[item_content['mime_type']]
        out = URIS.get(item_class)
        if out:
            out = out.format(extension=extension, **item_content)
        return out