comment that format field is for frontend backward compatibility

comfy_execution/jobs.py

@@ -41,6 +41,7 @@ def is_previewable(media_type: str, item: dict) -> bool:
     """
     Check if an output item is previewable.
     Matches frontend logic in ComfyUI_frontend/src/stores/queueStore.ts
+    Maintains backwards compatibility with existing logic.
 
     Priority:
     1. media_type is 'images', 'video', or 'audio'
@@ -50,7 +51,8 @@ def is_previewable(media_type: str, item: dict) -> bool:
     if media_type in PREVIEWABLE_MEDIA_TYPES:
         return True
 
-    # Check format field (MIME type)
+    # Check format field (MIME type). 
+    # Maintains backwards compatibility with how custom node outputs are handled in the frontend.
     fmt = item.get('format', '')
     if fmt and (fmt.startswith('video/') or fmt.startswith('audio/')):
         return True
@@ -159,6 +161,7 @@ def get_outputs_summary(outputs: dict) -> tuple[int, Optional[dict]]:
         if not isinstance(node_outputs, dict):
             continue
         for media_type, items in node_outputs.items():
+            # 'animated' is a boolean flag, not actual output items
             if media_type == 'animated' or not isinstance(items, list):
                 continue