Adding editor styles through plugin

Both solutions don’t work for some reason, they give me a 404 file not found while the path to the file is 300% correct.

This is just copy/paste from the WordPress Codex and all I want to do is simply add some editor styles to my theme through a plugin.
Yes I tried those functions one by one not both at the same time.

function my_editor_styles() {

	$editor_styles = plugins_url( '/assets/css/editor_styles.css', __FILE__ );

	add_editor_style( $editor_styles );

}
add_action( 'after_setup_theme', 'my_editor_styles' );

// add extra editor CSS styles
function my_editor_styles( $mce_css ) {

	if ( ! empty( $mce_css ) )

		$mce_css .= ',';

	$mce_css .= plugins_url( '/assets/css/editor_styles.css', __FILE__ );

	return $mce_css;

}
add_filter( 'mce_css', 'my_editor_styles' );