Just to check if it was a setting in the HKCR\.bmp key which caused this, I backed it up and deleted it.
After that, I backed up HKCR\.jpeg too and renamed it to HKCR\.bmp, so that it should handle .bmp files exactly the same way as .jpeg files.
Unfortunately this didn't work either.
My guess is that the M$ Bastards® baked it in the OS somehow...
or are there other places where information like this could be stored?