It is so easy to forget to add the relevant attachment to an email when you are in a rush or have other things on your mind!
It may be worth adding the attachment before you write the body of the message. However, if you are using Outlook 2013 or 2016 and you include the words attachment or attached in the body of the email and then send the message without attaching the file, you will receive a message asking whether you have forgotten to attach a file. How useful is that!
Also when you go to the Attach File dropdown, you will now see a list of recently used files to easily attach instead of having to browse for the file.