An error occurred while saving the commentAndrew Hill commented
This is supported, but it's done so in a very non-standard way.
For some reason the developer has chosen to require that you press-and-hold on an attachment in order to drag it.
This breaks the normal/expected behaviour for drag & drop, so many users can't find it, or regularly take a couple of attempts at such a simple action.
To the developers, please remove the delay, it's an affront to Mac OS X's UI standards!