> If accepted, the file is saved with the wrong encoding

"accept" means click on the "OK" button. You have to click on the 
biggest button "Edit my file" to create a valid file.

The problem here is the text in the dialog but also the possibility to 
create an invalid file. 

Two solutions:
 - don't ask user for a confirmation (always insert the header)
 - replace "OK" button by "Cancel" button: the user wants maybe an 
ASCII only file but inserted by error an unicode character (eg. non 
breaking space!)