Attaching a file is slightly different depending if you are using the installed version of Outlook or if you are using the browser-based Outlook Web App:
Outlook Application (Installed on Computer)
Microsoft Support instructions: Attach files or insert pictures in Outlook email messages
- Open Outlook

- Create a new email, or open the email in which you would like to add the attachment.
- Click the Attach File at the top of the window and select the file to attach.
Once you attach it successfully, the attachment will show below the subject line.
Learn more
To learn more about attaching files different ways, see Microsoft support instructions: