#1485151 was solved after private talks with ALEC, so now new bug waiting for some action ...
http://trac.roundcube.net/ticket/1485311
The common office apps formats (m$ and openoffice) are misdetected by libmagic library and that means that roundcube sends such attachment with wrong Content-type.
I'm proposing to use plain filename extension based type detection for commonly misdetected formats. Example patch attached to bugreport.
Opinions?