Extension should be scdoc.
The X means SC is now running on Parasolid instead of acis.
DO you have SPEOS installed. The easiest way is to load Speos, go to options for Optical Simulation and change it from Parasolid to Acis and Close
Then SC will be back to Acis
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Or try this:
set user environment variable SPACECLAIM_MODE. Set a value of 1. Then launch spaceclaim and save a geometry file. See if the extension stays as *.scdoc