Actually, just tested this before committing and the text doesn't fit the box.
If you have a Blender build, the DMG is created with this command:
./release/darwin/bundle.sh --source ../build_darwin/bin/ --dmg test.dmg
My finder preferences have "Show all filename extensions". Still would be good to change the box to be bigger to accommodate that preferences, when searching google images for e.g. "dmg background" it seems to be relatively common to show the extension.