I just purchased AutoCAD LT 2015 upgrade for Mac.  After purchase I was able to download a file but when I open I get a Warning  The following disk images couldn't be opened Reason "Image not recogonized"

My OS is:

OS X Yosemite 

10.10.1

Macbook Pro (Retina, Mid 2012) 

File size of Autodesk_AutoCAD_LT_2015_English_Mac_OSX.dmg = 781,678,998 (781.7 MB)