This AddOn allows you to replace a file rather than have it renamed. This can be useful if you have a something like a PDF menu that you want to update every day/week/month. Just attach the file making sure it has the exact same name and send the email. Postie will replace the file rather that the WordPress default of renaming the new file to something unique.
You can specify the name of the file or an extension if you want to restrict Postie to just replacing certain files or file types. That way you can use Postie normally as well with worrying that your images will get replaced.