Optimize conversation when uploading attachments (#4964)

### What problem does this PR solve?

#4929

### Type of change

- [x] Performance Improvement

api/db/services/dialog_service.py

@@ -204,9 +204,6 @@ def chat(dialog, messages, stream=True, **kwargs):
     attachments = kwargs["doc_ids"].split(",") if "doc_ids" in kwargs else None
     if "doc_ids" in messages[-1]:
         attachments = messages[-1]["doc_ids"]
-        for m in messages[:-1]:
-            if "doc_ids" in m:
-                attachments.extend(m["doc_ids"])
 
     create_retriever_ts = timer()