that depends, there are 2 scenarios for sending an attachment:
1) if you only want to attach the file to the email, but not save it to the server:
In the Universal Email settings, go to the attachments settings section, select the Submitted File option and select the file field from your form
2) If you want to save the uploaded file to the server and send it as an attachment:
In the archived documentation section of the universal email support page, see the "Sending Email Attachments with Digital File Pro and Universal Email" tutorial
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