Attachment fields may be defined in the Field List, and subsequently added to the Default form to display a field for uploading an attached file.
Select the Module link in the top left banner then select the appropriate module name link.
Select the Forms tab.
Then select the Field List button "Create/Update Fields".
Select Add Field button in the top right.
Defined the Internal & System Names and select the Type dropdown as Attachment, then select OK.
Select Save on the Field List form.
Then select the Default form.
Select the palette dropdown heading, then Attachments.
Drag and Drop the Attachment field to the from the palette on the right.
Select Save in the top right.
Visit the form and verify the field is appearing as anticipated