Add DOCX Mime Type

add_filter( 'upload_mimes', 'my_myme_types', 1, 1 );
function my_myme_types( $mime_types ) {
  $mime_types['doc'] = 'application/msword';     // Adding .doc extension
  $mime_types['docx'] = 'application/vnd.openxmlformats-officedocument.wordprocessingml.document'; // Adding .docx extension

  unset( $mime_types['xls'] );  // Remove .xls extension
  unset( $mime_types['xlsx'] ); // Remove .xlsx extension

  return $mime_types;
}

// Add WP Add Mime Types plugin to work