Currently you click a file attachment and it might say "Preiew for this file type is not supported" and the email client displays a Download button. In other mail clients, there is usually an accompanying "Open" button, which saves the file to a temporary location on the harddrive and then opens it with the system's registered associated program.

On windows, they use the "open" utility, on linux it's xdg-open, etc, you can avoid having to save a file with a name to a location and then find that file to open it by just having a button that says "Open" right away. This would speed up my workflow.