0.4.0, the latest version of Parole includes two new files:
The name 'extension' is pretty generic and might result in clashes when other packages provide an icon with the same name.
As long as other themes don't provide an 'extension' icon (it's not part of the xdg icon naming spec) and it's not supported to be themed, you should consider it renaming it to something more specific, e.g. 'parole-extension'.
Agreed, thanks for the bugreport, this should and will be changed!
Fixed in git master...
As suggested, parole-extension.png has replaced extension.png