LibreOffice on El Capitan Mac

When I click on a .dmg file LibreOffice grabs it, I have to use Open With to load the software application. I’m not sure if this is a bug, or I have something set up wrong.

can you help plse.

Try this:
Right Click|Get Info.

Half way down the Inspector panel is an “Open with:” section. It should show as “DiskImageMounter (default)”.

If it doesn’t, select DiskImageMounter in the drop down chooser, and click on “Use this application to open all documents like this one.”