My attachments disappear in Outlook when I forward them, how is this possible?

This is a known issue with Outlook, caused by a security update from Microsoft. (See KB 4011626 to see which update is involved)

To solve this problem, Microsoft has released another update that solves this problem in most cases.

This update solves an issue that removes some attachments in plain text messages when you forward the message.

(see: https://support.microsoft.com/help/4011123 )