The latest version of WebKit wasn't stable enough. We rolled back to the WebKit we used in the previous version of Mailplane 3.0. Sorry for the crashes.
We removed the Attachment Drawer because we weren't happy how it turned out. It was irritating and not reliable enough.
The new approach works basically like Mailplane 2 did it before. It attaches files immediately and provides support dependent on the file type. This version contains only some initial functionality. We work hard to improve this during the next few weeks.