SELECT 
  cscart_product_files.*, 
  cscart_product_file_descriptions.file_name, 
  cscart_product_file_descriptions.license, 
  cscart_product_file_descriptions.readme 
FROM 
  cscart_product_files 
  LEFT JOIN cscart_product_file_descriptions ON cscart_product_file_descriptions.file_id = cscart_product_files.file_id 
  AND cscart_product_file_descriptions.lang_code = 'en' 
WHERE 
  1 = 1 
  AND cscart_product_files.product_id = 339754 
  AND preview_path != '' 
  AND cscart_product_files.status = 'A' 
ORDER BY 
  cscart_product_files.position asc, 
  cscart_product_file_descriptions.file_name asc

Query time 0.00097

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.80"
    },
    "ordering_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_product_files",
            "access_type": "ref",
            "possible_keys": [
              "product_id"
            ],
            "key": "product_id",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "50.00",
            "cost_info": {
              "read_cost": "1.00",
              "eval_cost": "0.10",
              "prefix_cost": "1.20",
              "data_read_per_join": "788"
            },
            "used_columns": [
              "file_id",
              "folder_id",
              "product_id",
              "file_path",
              "preview_path",
              "file_size",
              "preview_size",
              "agreement",
              "max_downloads",
              "total_downloads",
              "activation_type",
              "position",
              "status"
            ],
            "attached_condition": "((`webmarco`.`cscart_product_files`.`preview_path` <> '') and (`webmarco`.`cscart_product_files`.`status` = 'A'))"
          }
        },
        {
          "table": {
            "table_name": "cscart_product_file_descriptions",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "file_id",
              "lang_code"
            ],
            "key_length": "9",
            "ref": [
              "webmarco.cscart_product_files.file_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.50",
              "eval_cost": "0.10",
              "prefix_cost": "1.80",
              "data_read_per_join": "400"
            },
            "used_columns": [
              "file_id",
              "lang_code",
              "file_name",
              "license",
              "readme"
            ]
          }
        }
      ]
    }
  }
}