On a new Windows 8 computer KindlePreviewer.exe is the system default for mobi files. In Calibre I've unchecked the use internal viewer box in preferences->behavior marked mobi. I've also unchecked the box marked epub. The system default for epub is sigil and when I click on the format, epub in the Book Details pane sigil opens the selected book, but when I click on mobi nothing happens? Both sigil and KindlePreviewer work fine in Windows Explorer.