Since you are using Outlook 2010, you can't open and save them directly back. That was removed for at least two reasons: because the securetemp directory is deleted when Outlook closes, and because users couldn't find the saved file. There are quite a few threads about it - just google around a bit and you'll find them.
So, you have two choices. You can use a different email application that does allow attachments to be opened and saved back to the original email, or you get used to saving the attachment, editing it, and then adding it to your new/forwarded email.