When sending an email you may attach files to it using the Attach File tool which you can drag onto your canvas from the Action Flow Toolbox.
The attachment of files is made at runtime, before sending the email. Therefore, the Attach File tool is only available in the Preparation of Emails.
Typically the design for attaching files is as follows:
Add input parameters to the Email to pass runtime values related with the files to attach to the email;
In the Preparation of the Email, use the input parameters to get the files and attach them using the Attach File tool.
Attach Files Properties | About Emails