FastDMG is a free macOS utility to mount
.dmg and other disk images quickly, without any unneeded nonsense. It is a very fast and reliable alternative to Apple's annoying DiskImageMounter.
FastDMG is actually a minimal wrapper around the
hdiutil command line tool that ships with macOS and therefore uses Apple's private DiskImages framework indirectly. It is free, open source software and has a home on GitHub. If you like it, feel free to make a donation.
.dmgfile and press Cmd-I to show the Finder's Get Info window
FastDMG will then take care of mounting
.dmg disk images when they're opened in the Finder. You can do the same for
.smi and any other disk images supported by FastDMG.