def typed_subpart_iterator(msg, maintype='text', subtype=None):
    """Iterate over the subparts with a given MIME type.

    Use `maintype' as the main MIME type to match against; this defaults to
    "text".  Optional `subtype' is the MIME subtype to match against; if
    omitted, only the main type is matched.
    """
    for subpart in msg.walk():
        if subpart.get_content_maintype() == maintype:
            if subtype is None or subpart.get_content_subtype() == subtype:
                yield subpart
