To manage attachments on the selected file click on the button 
Upload an attachment
Click on the 
button in the upper right corner to reveal an upload dialog
Click "Click here to add a new file" and browse to the file you wish you attach and click Open
Delete an attachment
Click on the attachment you wish to delete.
Click on the button 
in the bottom right hand corner to delete the selected attachment.
Set attachment as primary
Click on the button 
in the bottom right hand corner to make the selected attachment the primary.
The primary attachment is shown in the 