I'm not sure if it'll work for everyone, but I found a faster workaround that has worked well for me. There are also some other issues I've found with iClone6, which I've found the workarounds as well, so here goes nothing.
Instead of going through each file that need file extension changes, what I've done is to batch change entire folders.
You'll want to download this
'Extension Changer' first. When prompted to install, make sure you choose "Custom" and deselect "Change homepage to Yahoo" and such, which will affect your browser. You'll also be prompted to install some program, make sure to decline that as well, as most of these software are junks at best.
After installing the 'Extension Changer' program, double click and open the program. Then navigate to the folder of the content that needs the extension changed. If the contents are props, then type "iProp" (without quotes) in the "Change To" section of the program. Select all the files that need changed and drag them to the tiny square right next to where it says "File Name" "File Type". Give it a second or two and it's done.

There are couple of things that you'll find inconvenient about the Extension Changer. One, it doesn't recognize multiple folders well. So you can drag several folders if you want, but you'll find that some folders had not been modified. I like to be sure, so what I've done is to go into each folder that needs changing, "Select All" and make sure each and every folder got modified. Two, you'll have to start the program each and every time it finishes a batch. But in the end, they're only minor inconvenience.
Other Problems I've found with iClone6:
1 - Default iProp 'Jeep' isn't included with it. You'll have to copy and paste from iClone5 content folder to your iClone6 iProp folder. You can find 'Jeep.iProp' in the Drive:\Reallusion\Template\iClone 5 Template\iClone Template\Props folder of iClone5.
2 - Home, Stage, Outdoor folders are missing in iClone6. Once again, copy and paste from your iClone5 folder. The folders are located in the Drive:\Reallusion\Template\iClone 5 Template\iClone Template\Props\iProps folder.
3 - If you've purchased the content packs 'Evil Force - Volcano of Doom', 'Famous Movie Scenes - Times Square', 'Films and Props Vol.1', you'll notice that they are not found in the 3D Scene section of your iClone6. To fix this, you'll need to navigate to your content folder (Ex. - Drive:\Reallusion 6\Template\iClone 6 Template\iClone Template\3D Scene), then change the extension from vns to iScene.
I've done some other things to make iClone6 behave normally, but this is all I can remember from the top of my head. Hopefully this might be helpful to some people. That's it folks, happy new year!