SELECT 
  object_id, 
  filename, 
  filesize, 
  category, 
  last_updated, 
  d.description as 'description' 
FROM 
  cscart_attachments as a 
  LEFT JOIN cscart_attachment_descriptions as d ON a.attachment_id = d.attachment_id 
WHERE 
  a.attachment_id = 21015 
ORDER BY 
  d.description ASC

Query time 0.00023

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "filesort": {
      "sort_key": "d.description",
      "temporary_table": {
        "table": {
          "table_name": "a",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["attachment_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        },
        "table": {
          "table_name": "d",
          "access_type": "ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["attachment_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      }
    }
  }
}

Result