Refa: migrate document preview/download to RESTful API (#14633)

### What problem does this PR solve?

migrate document preview/download to RESTful API

### Type of change
- [x] Refactoring

docs/references/http_api_reference.md

@@ -6879,14 +6879,18 @@ Failure:
 ##### Request example
 
 ```bash
-```
+curl --request GET \
+     --url 'http://{address}/api/v1/documents/{doc_id}/download?ext=pdf' \
+     --header 'Authorization: Bearer <YOUR_API_KEY>' \
+     --output ./downloaded_attachment.pdf
+```
 
 ##### Request parameters
 
 - `doc_id`: (*Path parameter*), `string`, *Required*
   The document ID whose attachment should be downloaded.
 - `ext`: (*Query parameter*), `string`, *Optional*
-  A file extension hint specifying the response's Content-Type. Defaults to `"markdown"`. Available values:
+  A file extension hint specifying the response's Content-Type. Defaults to `"markdown"`. Available values:
   - `"markdown"`
   - `"html"`
   - `"pdf"`
@@ -6896,15 +6900,15 @@ Downloads a runtime attachment previously uploaded via the [Upload document](#up
 
 #### Response
 
-Success:
+Success:
 
 Returns the file content as a binary stream with the relevant Content-Type header.
 
 Failure:
 
 ```json
-{
-    "code": 500,
+{
+    "code": 500,
     "message": "Internal server error"
 }
 ```