WordPress at a glance
action is not described

media_buttons action-hook . WP 2.5.0

Fires after the default media button(s) are displayed.

Usage

add_action( 'media_buttons', 'action_function_name_5890' );
function action_function_name_5890( $editor_id ){
	// action...
}
$editor_id(string)
Unique editor identifier, e.g. 'content'.

Where the hook is called

_WP_Editors::editor()
wp-includes/class-wp-editor.php 227
do_action( 'media_buttons', $editor_id );

Where the hook is used (in WP core)

wp-includes/default-filters.php 526
add_action( 'media_buttons', 'media_buttons' );