This guide describes how you create a service menu to allow you to right click a file in Dolphin and create a new mail with the file as an attachment.
kde4-config --path services
On my SuSE 13.2 system I get the following output:
The first path shows where your personal ServiceMenus directory should be located. Check if there already is such a directory there, if not create it.
For Plasma 5 there is no such tool AFAIK but on my SuSE Leap 42.2 system it is
In the ServiceMenus directory, create a new file called sendto.desktop with the following content:
[Desktop Action SendToMailRcp]
Exec=thunderbird -compose attachment="file:///%f"
If you use kmail instead of thunderbird, replace the last two lines with:
Exec=kmail --attach %F
For Plasma 5 you then need to run kbuildsycoca5 from the command line, or log out and log in again.
Start Dolphin and right click a file. You should now have a "Send To" submenu with a "Mail Recipient..." entry.