I haven't looked at that action in a few months, but last I looked it's just a mistake in the object. 'Save Attachment' calls 'Save Attachments' and is supposed to pass over the File Name to be used to limit the attachment download to the specific name you give it. It was never intended to rename the attachment as it is being downloaded. I agree that this is annoying, but it's not a very difficult change to make if you want it to rename the file during download. Just copy the action and edit the code stage so that it gives a new name to the file before saving.
Without editing the code stage or without creating your own action, the way Michal describes is the standard way to use it. In my opinion, the easiest way to do this while also being safe is to have a temporary folder that you always delete all files from before downloading attachments to it. Then you grab whatever attachment from that folder/directory that you want and move/rename the file to the folder you really wanted to download it to.
Dave Morris, 3Ci at Southern Company
