I was checking Twitter for questions about Outlook yesterday when I came across this one from smelibari.
Outlook doesn’t retain attachments on replies, presumably because the reply is going back to the person that sent the attachment(s). Clearly they must already have them or they couldn’t have sent them to you. So, while I’m not sure what the use case is for replying with attachments, the solution is pretty simple. Use a macro to create the reply, then add all the attachments from the original message to the reply. The macro will work with both an open message or a message selected in a folder. Of course the open/selected item must be an email. This solution should work with Outlook 2007 and later.