fix: handle unicode filenames in external document loader

Files with special characters in their names (e.g., ü.pdf) caused issues since HTTP headers only allow Latin-1 characters.
This change URL-encodes `X-Filename` before adding it to request headers, preventing failures when uploading or processing such files.

Fixes: #17000

backend/open_webui/retrieval/loaders/external_document.py

@@ -1,6 +1,7 @@
 import requests
 import logging, os
 from typing import Iterator, List, Union
+from urllib.parse import quote
 
 from langchain_core.document_loaders import BaseLoader
 from langchain_core.documents import Document
@@ -37,7 +38,7 @@ class ExternalDocumentLoader(BaseLoader):
             headers["Authorization"] = f"Bearer {self.api_key}"
 
         try:
-            headers["X-Filename"] = os.path.basename(self.file_path)
+            headers["X-Filename"] = quote(os.path.basename(self.file_path))
         except:
             pass