Using add_theme_support inside a plugin

I’ve created a custom post type as a plugin and released it into the repository. One of the core features involves using a featured image. I’ve added thumbnail to $supports in register_post_type(), so the meta box shows up in the Administration Panel. I’m also hooking into after_setup_theme and calling add_theme_support( 'post-thumbnails' ), but I don’t think it’s taking affect.