有多个操作可用。演示：

add_action( \'upload_ui_over_quota\', \'wpse_78085_callback\' );
add_action( \'pre-upload-ui\', \'wpse_78085_callback\' );
add_action( \'pre-plupload-upload-ui\', \'wpse_78085_callback\' );
add_action( \'post-plupload-upload-ui\', \'wpse_78085_callback\' );
add_action( \'post-upload-ui\', \'wpse_78085_callback\' );


function wpse_78085_callback()
{
    # see wp-includes/media-template.php
    print \'<pre>\' . current_filter() . \'</pre>\';
}