What I am really missing is a extension for creating small jpeg attachments by dropping some big jpeg files at a new message.
I think of a simple options dialog to turn that feature on or off and to set the prefered size (resize to fit into bounding-box: width x height pixel) and the prefered jpeg quality. There also could be an option to keep or remove metadata (e.g. exif information).
I am sure there are many people out there looking for a feature like that.
A simple way to add an image that is stored in the clipboard as image-attachment (!) would be really useful, too.