Most email apps I've used me to drag and drop a file from the app straight into another app or website. So I'd expect a file can be pulled from Missive, for example, and dropped straight into an upload prompt on Dropbox or WordPress website.
In the current version, however, drag and drop only works from Missive to Finder. Drag it anywhere else and the other app doesn't recognize that I'm trying to drop a file on it. So I have to drag from Missive to Finder, then grab and drag from Finder to the app I need the file in, then go back and delete the app out of Finder because I didn't really need it there in the first place.
Would love to eliminate all of those extra steps :D