I figured this out today for both eM Client and Thunderbird. Really works slick on W7 and W10.
Simply put an eM Client shortcut into the Send to folder. When you right click a file and choose Send To: You simply choose eM Client. A new message will open with your file attached. Just add your recipients and type your email. NO Registry work needed.
It only works with one file. However, once the new message opens you can attach more files. It’s better than nothing IMHO.
Path to Send To folder:
C:\Users"type your user name here"\AppData\Roaming\Microsoft\Windows