The relevant specification is RFC 6266 (http://greenbytes.de/tech/webdav/rfc6266.html), and it says that unknown disposition types should be treated as "attachment" (see http://greenbytes.de/tech/webdav/rfc6266.html#rfc.section.4.2).
With respect to the example you cite: no, IE does support "attachment" (and has forever). The problem over there might have had to do with nonsense caching response headers.