quick and dirty:

1. sudo -i # getting root
2. cd /usr/share/mime/packages/
3. cp keepassx.xml truecrypt.xml #(just keepassx because is for security too)
4. edit truecrypt.xml # with usefull content -> application/x-truecrypt
5. update-mime-database /usr/share/mime
6. cd /usr/share/application
7. edit defaults.list # -> insert truecrypt with truecrypt.desktop
8. cp keepassx.desktop truecrypt.desktop
9. edit keepassx.desktop # with usefull content -> e.g. set Exec=truecrypt
10. killall nemo
11. start nemo and double click a *.tc file -> truecrypt will promt with asking the password