On Debian Jessie (testing) the default ‘Send as Email Attachment’ context menu item uses KMail, and only KMail, even if you’ve configured your desktop to use Icedove (Thunderbird) as your default mail client.

So you can remove the broken ‘Send as Email Attachment’ from Dolphin by opening Dolphin then:

Settings -> Configure Dolphin… -> Services -> uncheck ‘Send as Email Attachment’

Then to create a replacement feature that uses Icedove edit:

~/.kde/share/kd4/share/icedove_attachment.desktop

And enter:

[Desktop Entry]
Type=Service
Actions=attachToEmail
Encoding=UTF-8
ServiceTypes=KonqPopupMenu/Plugin,all/allfiles
ExcludeServiceTypes=application/x-iso,kdedevice/*,inode/directory

[Desktop Action attachToEmail]
Exec=icedove -compose "attachment='$(echo %F | sed 's/\\ \\//,\\/\\//g')'"

Name=Attach to Icedove
Name[it]=Invia E-mail con Icedove
Name[es]=Enviar adjunto con Icedove
Name[de]=Als Anhang mit Icedove verschicken
Name[pt]=Anexar ao Icedove E-mail
Name[pt_BR]=Enviar arquivo(s) como anexo(s)
Name[fr]=Envoyer avec Icedove
Name[nl]=Voeg toe als bijlage aan Icedove
Name[pl]=Wyślij jako załącznik Icedove
Name[ru]=Отправить с помощью Icedove
Name[cz]=Odeslat jako přílohu Icedove

Icon=/usr/lib/icedove/chrome/icons/default/default16.png

And make sure your .desktop file is executable:

 $ chmod +x ~/.kde/share/kd4/share/icedove_attachment.desktop