[NOTICKET][document] make more clean output for error message unspported file

src/pipeline/document_pipeline/document_pipeline.py

@@ -32,7 +32,7 @@ class DocumentPipeline:
         if file_type not in SUPPORTED_FILE_TYPES:
             raise HTTPException(
                 status_code=400,
-                detail=f"Unsupported file type. Supported: {SUPPORTED_FILE_TYPES}",
+                detail=f"Unsupported file type. Supported: {', '.join(SUPPORTED_FILE_TYPES)}",
             )
 
         blob_name = await blob_storage.upload_file(content, file.filename, user_id)