acolson

add_filter( 'mime_types', 'aco_extend_mime_types' );

function aco_extend_mime_types( $existing_mimes ) {

  // Add webm, mp4 and OGG to the list of mime types
  $existing_mimes['webm'] = 'video/webm';
  $existing_mimes['mp4']  = 'video/mp4';
  $existing_mimes['ogg']  = 'video/ogg';

  // Return an array now including our added mime types

acolson

add_filter( 'manage_edit-post_columns', 'aco_last_modified_admin_column' );

// Create the last modified column
function aco_last_modified_admin_column( $columns ) {
  $columns['modified-last'] =__( 'Last Modified', 'aco' );
  return $columns;
}

add_filter( 'manage_edit-post_sortable_columns', 'aco_sortable_last_modified_column' );