In QMimeType class, the “rtf” extension is associated to the “text/richtext” mime type, instead of the correct “text/rtf” mime type.
The “text/richtext” was a predecessor of the Rich Text Format we currently use, and it's not compatible with it. See http://en.wikipedia.org/wiki/Rich_Text_Format for more details.
Easy to fix: simply change “text/richtext” with “text/rtf”.