there are 2 ways to attach files to an email
1) Dont use the file upload behavior and select the Submitted file option on the attachments tab of the UE behavior
2) Use the file upload behavior to save the file to the server. Then on the attachments tab of the UE behavior, select the enter value option.
Click the lightning bolt icon and expand the Uploaded Files binding group, select the Server Directory binding. Copy the code for the server directory binding, then select the Server file name binding. lastly, copy the server directory code in front of the server file name binding code.