upload File: update operation in mode field is not working

If the upload succeeds, that means the file was indeed updated (or the contents weren't changed). Some ideas of things to check:

1. Did you really send updated content via the API? (E.g. maybe you're editing the wrong file, not the one you're actually uploading.)
2. Are you looking at the right file when you're trying to verify that the upload succeeded? (E.g. maybe the file is being updated but you're checking the wrong file.)

As a sanity check, you might try logging the file size and timestamp in your code right before upload. That way you can double check that you're uploading what you expect.