Office 365 (Outlook 2013 / 2016 for Win) - Add an Attachment to an Email Message
To share a file, you can attach it to your message. You can also attach other Outlook items, such as messages, contacts, or tasks.
Create a message, or for an existing message, click Reply, Reply All, or Forward.
You can drag your attachment into the body of the message, OR
In the message window, click Message > Attach File.