fix: remove incorrect .dat extension from async item file path

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/services/async_item_service.rs

@@ -83,7 +83,7 @@ impl AsyncItemService {
             .unwrap_or_else(|| "application/octet-stream".to_string());
         
         // Open the file for streaming
-        let file_path = self.data_dir.join(format!("{}.dat", item_id));
+        let file_path = self.data_dir.join(item_id.to_string());
         let file = tokio::fs::File::open(&file_path).await
             .map_err(|e| CoreError::Io(e))?;
         let mut buffered_file = tokio::io::BufReader::new(file);