Basically in all linux machines if we want to send some files then using mail,then on right clicking we will be getting the send to option,so by default it will be evolution.
For thunderbird mail client users we have to install some plugins inorder to have the send to option
Download the plugings from here http://www.novadevel.com/m/ifeghali/thunderbird/
you can get the plugin source or it u are a debian user u can get the deb file.
Finally your send to will look like this