I still say that Dolphin is by far the best file manager out there right now. What I show you in the image, yes, Nautilus and maybe others do it too, but it is an option that at least I find quite comfortable 
I have to send some files/documents by email, and having to open a new email, click the attach button and search for the file, I find it just too annoying 
The author of this is danux, and to have this here are the steps:
1. Open a terminal.
2. In it write the following and press [enter]:
cd $HOME && wget http://kde-apps.org/CONTENT/content-files/122832-thunderbird_attachment.desktop
3. The file «122832-thunderbird_attachment.desktop«, They must copy it to ~ / .kde4 / share / kde4 / services and ready. Close and reopen Dolphin (File Browser) and you will see the option 
The author made a modification to the file, making the icon show only if Thunderbird is 64bits, if you use 32bits (like me) instead of putting the line from step # 2, put this one:
cd $HOME && wget https://blog.desdelinux.net/wp-content/uploads/thunderbird_attachment.desktop
And well, nothing more to add.
Any doubt or question, problem or whatever they tell me.
Greetings 
Attach to thunderbird at KDE-Apps.org
