def get_content_disposition(mimeobj):
    """Return the message's content-disposition if it exists, or None.

    Backport of py3.5 :func:`~email.message.Message.get_content_disposition`
    """
    value = mimeobj.get('content-disposition')
    if value is None:
        return None
    return str(value).partition(';')[0].strip().lower()
