First, you need to have version 2.5.2 or later of DT Register. When this version is installed, it should have created a new folder here:
[your_joomla_site]/images/dtregister
Make sure that folder exists and that it is writable.
Then in Event Management, you'll find a new area for the file attachments labeled as "Attach Files to Thank You Email". Click the BROWSE button to locate the file on your computer. Selecting the file will automatically add it to a list of files seen right there in this area. Next to each one is a "DELETE" button. After loading your files, click the SAVE button to save the event. These files will be loaded into the /images/dtregister folder.
Now whenever a new registration comes through for that event, the uploaded files will be attached to the email that is sent to the user. These files are loaded PER EVENT.