fix: add status_code 304 (#6000)

Co-authored-by: crazywoola <100913391+crazywoola@users.noreply.github.com>
Co-authored-by: crazywoola <427733928@qq.com>

api/core/file/message_file_parser.py

             }
 
             response = requests.head(url, headers=headers, allow_redirects=True)
-            if response.status_code == 200:
+            if response.status_code in {200, 304}:
                 return True, ""
             else:
                 return False, "URL does not exist."