It's a great tool and this should save many disputes from occurring, but in the case you point out about a file that is used on the Spanish Wikipedia (which has disabled local uploads), then such files, if extracted to the English Wikipedia, will only be linkable through an interwiki link to the English Wikipedia and cannot be viewed as a thumbnail on the Spanish Wikipedia.

So your tool will only work for the Wikipedia projects with local uploads enabled, but still, it's definitely better than what we have now, which is a mess.

Jane