You’re missing an underscore:
add_action('add attachment', 'bZive_generate_AlphanumericID', 10, 3);
Needs to be:
add_action('add_attachment', 'bZive_generate_AlphanumericID', 10, 1);
I also changed the number of accepted arguments to 1, because add_attachment only has 1 and your callback function is only using 1 anyway.