Can standard "L shaped" shelf brackets be mounted from the top and still be strong?

i'm planning to mount shelves for 30pound bookshelf speakers and i'd like to have the bracket above the shelf, so it isn't seen and also because in one location, there isn't room to put a bracket below the shelf. this is the type of bracket i'm thinking about using: http://www.homedepot.com/h_d1/N-5yc1vZ1xgh/R-202034279/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053