Leave them as they are. These keys, or rather their sub-keys OpenWithList
and OpenWithProgids include details of programs that you have configured
to open these various file types using the right click OpenWith dialogue.
What may have happened is that you at one time configured your system such
that program A could open files of type B and have since deleted program A
whilst leaving the data in the FileExt key for files of type B.
If this really worries you then carefully remove by hand the unwanted
entries using Regedit but I would not recommend any "registry cleaner".
Do NOT post your problem in someone elses thread.