Office 365 (Outlook 2013 / 2016 / 2019 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.
For additional instructions including inserting images into the body of a message and attaching business cards and calendar events, please see the following Microsoft help article: https://support.office.com/en-us/article/Attach-files-or-insert-pictures-in-Outlook-email-messages-BDFAFEF5-792A-42B1-9A7B-84512D7DE7FC.