With Attachments, there are 2 ways to work:
1) Not Using File Manipulation to upload the file. If you don't add the Upload behavior to the page, on the attachments section of the UE wizard, select the Submitted File option and select the File Field.
2) using file upload to save the file to the server. If you use file upload to save the file to the server, select the Enter Value option and click the lightning bolt icon. Expand the Uploaded Files (your file field) binding group and select the server directory binding, copy the binding code for the server directory binding, then select the Server File Name binding and click OK.
in the enter value, place your cursor before the server file name binding code and paste the server directory binding code.
make sure you are using the correct work flow for setting up attachments in UE based on whether you use file upload to save the file to the server or not.