After banging my head for nearly 4 hours, I''ve learned that MSIE is really picky (what else is new?) I was trying to send a pdf file as an attachment. ActionPack 9.0 removed a couple of headers that it sent when it saw the client was MSIE. Those headers were: @headers[''Pragma''] = "" @headers[''Cache-Control''] = "" Now I''m not exactly sure what those headers mean (I will be reading the spec later) but they now allow the pdf file to download. Just a tid bit of knowledge in case any one else is loosing their mind.