Hello!

Just wondering...didn't think about that at all in the past...

Why is actually a light cartridge case made from silver paper attracted both to a positively charged glass stick and to a negatively charged ebonite one?

Even objects with no net overall charge (i.e. neutral) can gain localised charges, e.g. the positive glass stick will 'pull' (attract) electrons from the paper towards the surface until the surface has a localised negative charge which then attracts the glass stick, the ebonite rod will 'push' (repel) electrons away from the surface of the paper so that there becomes a localised positive charge at the surface which then attracts the rod.

 

A simple experiment is to get a balloon and rub it vigorously against a woolen jumper (or your own hair will do) and then 'stick' it against the wall.

Oh...I think I didn't get the initial question...I thought there is something special about the cartridge case that makes attraction to both possible...but I didn't think about a "sequence" of events - trying and getting the cartridge case nearer to the glass stick and then to the ebonite rod...I saw two independent experiments...and that was a silly mistake...That's why, I think the question is meant to be answered exactly the way you did it.