It is not actually necessary to include the file extension in the attachment name in VaultWiki 4, plus the attachment name does not have to match the correct file extension if it does include one. For this reason, the correct file extension is added to the download link so that the file can actually be read by a downloader's system using the correct mime type.
However, for attachment names that do include some sort of extension already, it would be better to remove the extension inherited from the name when generating the download link, in an effort to be both aesthetically pleasing and to avoid confusion if a user must manually choose a program to open the file. Thus, this issue is now Confirmed.