An attempt to repair the encoding mess a bit, when attaching files:

- Don't set the charset to us-ascii if it couldn't be found
- Give up on KEncodingProber, it doesn't work reliable enough.
Instead, add an encoding combobox to the file dialog.
Now it is the user's fault if the encoding is not correct :)

- Don't attempt to set a charset for a non-textual mimetype
- Don't show an empty string for the mimetype