Hello nlijkje, and welcome to Seven Forums.
You might also check the same registry location as above in Option Three, but start at the HKEY_LOCAL_MACHINE
locationinstead to see if you may have an entry there as well.
If not, then use the Find and Find Next
feature in regedit to search for the HideFileExt
DWORD to see if there may a stray entry in the registry set to 1
Hope this helps,