There seems to be a lot of people looking for an answer to replying to an email and include the attachment. Normally the attachment is not included in the reply. People who were using lotus Notes and moved over to Outlook 2010 were used to a button to include attachment when replying, there is no such button in Outlook.
There is the option to “include attachments with all replies” but, by design, this feature is turned off so that inboxes and the send process is not clogged up with unnecessary attachments.
There are a range of complex answers and workarounds to this issue, including downloadable VB Scripts, but if you need to reply occassionally with the original attachment a quick solution is to right click the attachment in the original email, select copy, then hit reply. In the reply email ctrl+V and the attachment will appear in the correct place.