To rename a lot of files at once I use The Bulk Rename Utility, see www.bulkrenameutility.co.uk It is free to use.
It is very useful when you upload or download a attachment split in more than 15 parts. Lots of vid in platinum section are that large.
Only extensions up to .015 are allowed to upload, so I add .zip after .016 and more.
With this utility I can add this to all the parts at once.
And the other way around I can remove it too with this utility.
The interface looks confusing the first time. However, just try and once used you will appreciate this utility for sure.
Have fun.