Fix encoding problem with `kmail --msg $PWD/file.txt`

If the file contains UTF-8, on a Linux sytem with UTF-8 locale,
the message composer was showing bad encoding due to the toLatin1()
call (how could this be correct for non-latin1 users anyway...).

The weird thing is that it basically reverts the last commit in
bug 403313, but I guess things changed in kmime or OTP or something
meanwhile.