This process has been obsolete since version 16.1.
Click Insert File button and select the corresponding file.
You can also drag and drop the file to the Drag and Drop panel of the Attach Files screen.
Click the Upload button. Note that you can upload 1 file only per section.
Here is how the it will look on Quote Template.
Save the changes.
To view the attachment, select the row and then click Download File button. The system will download it to your default Download folder.