Very good idea!!
Your extension is very useful to watch or download galleries of pictures.
I have a feature request to enhance your extension:
The extension is currently incrementing the number located at the extreme end of the url, if existing.
But often, the number is not at the end of the URL, for instance :
www.domain.com/test01.html => www.domain.com/test02.html etc...
www.domain.com/test01initial/ => www.domain.com/test02initial/ etc...

Could it manage incrementation of the first number detected starting from the right of the url?