I've been having trouble with my Mac since I installed Yosemite, where I open an .dmg file, and it would go to the Disk Utility, then I would have to open it from there. My MacBook running Yosemite did not do this, and would give the normal graphic to drag the file over applications to execute it.
Now I've upgraded to Sierra, and I get an alert that says "The document “Example.dmg” could not be opened. Disk Utility cannot open files in the “Disk Image” format". I try to right click the file, but it does not give me an "open with" only an "open" which gives the same error. Again, my MacBook just opens the installer program just fine. I verify the images on Disk Utility, and I get an "operation successful", but the file still won't open.
I am sure there is a way to get round the problem and open the .dmg files, but I would like to adjust the settings so that the installer opens as designed.