fix: send only the file name to Docling instead of the full storage path (#30357)

With the Docling content extraction engine, every conversion request
carried the server's full internal upload path (for example
/app/backend/data/uploads/<id>_report.pdf) as the multipart file name.
Docling only needs a bare file name, and a hosted Docling instance has
no business learning where Open WebUI keeps its files on disk.

The loader now sends the base name of the stored file, which is what
the MinerU, Datalab and Mistral loaders already do. Nothing else in
the request or the parsed result changes, verified against a capturing
mock server before and after.

Fixes #30352

backend/open_webui/retrieval/loaders/main.py

@@ -273,7 +273,7 @@ class DoclingLoader:
                 f'{self.url}/v1/convert/file',
                 files={
                     'files': (
-                        self.file_path,
+                        os.path.basename(self.file_path),
                         f,
                         self.mime_type or 'application/octet-stream',
                     )