The use case for this is file encryption or the PGP/Partitioned "attachment.pgp" if you rename an encrypted file after encryption you loose the Information which type the file is if you remove the original extension and the filename might differ.
This is important because filenames / filetype might be confidential. As the original filename is embedded anyway (have to make sure this also happens when kleopatra is used) we can use this.
As automatic renaming the output file can have security implications e.g. if the output then is an .exe or .dll or tries some kind of path attack is problematic, my idea would be to open a message box when the output file name differs from the original filename and then ask the user if the output file should be renamed to the original filename.