If the download was a .dmg file, right click on it > > Get Info > in the pop-up window choose Open with > diskimagmounter.app from the list. If the diskimagemounter.app is not listed, choose Other... then navigate to Macintosh HD > System > Library > Core Services to choose diskimagemounter.app. Click on the Change All... button so all .dmg files will be opened with diskimagemounter.app