7D file naming _MG_xxxx

My 7D started to name the raw files like _MG_xxxx.cr2 instead of IMG_xxxx.cr2.
So instead of the I, there is an underscore in front.

Is there a setting that I might have accidently changed that might have caused this?
It's not a big deal, but, well... I just hate that underscore in front.

Already tried to format my card, or make one shot, put the card into the computer and rename it so that I replace the underscore with a I, but it still is sticking to its _MG_xxxx naming theme.

Anyone have an idea?