This works for me and rather than spell it out I think a picture is worth a 1,000 words. I hope it works for you. Follow the Steps below and rename the default mail client to em client.
Setting it here works as well. 
Not if you already have Thunderbird installed. 
This an easier solution. Just add a shortcut to the Sendto Folder in AppData.
https://forum.emclient.com/emclient/topics/attachments-right-click-send-to
Yes, your shortcut method is a tried and tested option. Good if you have more than one email client, then you can choose which one to use by right-click on the file you want to send.
If you only want to use one, just set eM Client as the default. Besides the first icon in my screenshot above, if you scroll down and choose Set defaults by app , then set ALL the options for eM Client. Then regardless of how many email clients you have installed, it will choose eM Client. Of course, if you have deleted or altered the the original send to option from the appdata folder you mention in your other post, it is not going to work.