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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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