I made two different types of Assets: one for images and one for PDF. They works in 2 differents folder to avoid to mix them.
Using them inside entries it works good, if I try to upload something different I receive an error message.
If I click Assets on the main Menu (not inside entries) I'd like limit the upload to the asset I choose.
It means that I would like prevent to upload a pdf on asset images and vice versa and I'd like also to prevent uploading of any different file types.
Not sure if Im doing something wrong or its the natural behaviour of CRAFT.