The INSTRUME keyword value is 'Medium', in FITS files created by ASIImg_V1.9.1 (64bit), when connected to my ASI1600MC-COOL. I don't know if this issue is peculiar to the ASI1600MC.
In earlier versions of ASIImg, INSTRUME is correctly reported as 'ZWO ASI1600MC-Cool'.
I would appreciate feedback from a ZWO representative and|or anyone else who has experienced this issue.
Thanks,
/jim