The model-base match in get_asset_category_and_relative_path ignored
each bucket's extension set, so a file inside a registered base whose
extension the bucket cannot load (e.g. a .txt uploaded into
model_type:checkpoints) advertised a loader_path that no loader list
would ever resolve, while the tag side of the same stack already
excluded it. Apply the extension check used for backend tags (empty set
accepts any extension), keeping loader_path null exactly when no loader
can resolve the file.