My first attempt at a shelf bracket, it has 2 mounting holes on each axis, the inner hole is 5 mm and the maximum diameter of the screw head is 10 mm. I also created screw hole cover for better appearance, they just pop in there but adding a bit of glue can't hurt if they want to fall out :)
Length of each arm is 85 mm, so it is suitable for some smaller shelf with not super heavy load.