In Outlook Express you can choose whether the original message is included with a Reply. (Tools|Options|Send) If you check "Send original message with reply" the complete sequence of all mail for the subject can be kept together in the latest pass when you're discussing a subject, so it sometimes is a "feature" and not a problem. If you uncheck the box, only your reply gets sent. My recollection is that attachments are omitted with a "reply," or you can manually remove them and attach other items if wanted.
You can also choose to "Forward" the received mail back to the sender, and can include the attachments that were with the original message you received.
If you receive/send unwanted "extras" it's because you or your correspondent(s) aren't using the intentional features of the program in the ways intended.
I assume that Outlook has the same capability, but haven't looked at it recently.