I am not able to attach the stl file here owing to its file type.
In Windows Explorer right click on file name and select Send to Compressed (zipped) Folder.
Attach resulting *.zip file here.
I had no trouble opening in 2013.
I don't have access to 2012 to test.
It would seem the STL importation/exportation became corrupt after installing Mechanical Simulation. I have not tried to uninstall that to test the theory (and really don't won't to). Whether it is zipped, unzipped, assign .stl in the file name or let the program do it, the result is the same in 2012.
I have build 219, Release SP2
Check in the Add-In Manager that the module is loaded automatically at start-up in Inventor.
... Whether it is zipped, unzipped, assign .stl in the file name...
All of this doesn't make sense?