def multipart_content_type(boundary, subtype='mixed'):
    """
    Returns a dict containing a MIME multipart header with the given
    boundary

    >>> multipart_content_type('8K5rNKlLQVyreRNncxOTeg')
    {'Content-Type': 'multipart/mixed; boundary="8K5rNKlLQVyreRNncxOTeg"'}

    >>> multipart_content_type('8K5rNKlLQVyreRNncxOTeg', 'alt')
    {'Content-Type': 'multipart/alt; boundary="8K5rNKlLQVyreRNncxOTeg"'}
    """
    ctype = 'multipart/%s; boundary="%s"' % (subtype, boundary)
    return {'Content-Type': ctype}
