Fix webdatset multipart ext and npz
for a new dataset being released that has a field "url.txt" that is not read correctly as text; and another field "npz" that had an error

thsoe have been fixed in https://github.com/huggingface/datasets/pull/6904 and included in branch `datasets-2.19.1-hotfix`
Fix webdatset multipart ext and npz: for a new dataset being released that has a field "url.txt" that is not read correctly as text; and another field "npz" that had an error

thsoe have been fixed in https://github.com/huggingface/datasets/pull/6904 and included in branch `datasets-2.19.1-hotfix`