Please remove the .mdlp from the file name, so that it ends in just .dmg.
We are seeing this behaviour with users that have MathLab installed and we are investigating why this is happening. FWIW, this happens to other software DMG files as well, not just the Flash Player installer. For some reason, when users have MathLab installed, when the DMG file is downloaded, it appends the .mdlp extension to it. When users attempt to launch it, instead of being opened with DiskImageMounter, which is what OS X uses to open DMG files, it opens using MathLab and the result is what you posted. If this happens in the future, just delete '.mdlp' from the file name and DiskImageMounter will open the DMG, instead of MathLab.