this was a problem involving multiple parts, unit tests normally don't catches this. You need an integration/functional test and that can be much more time consuming to write for all "integrations" and code paths.
It was a single function that changed behavior after a refactoring. It did work where it did not need to, because the work was already done on the object. This is only hard if you don't test at all and can't already mock the object download/upload or don't have pure functions.
In this case it might have helped, I didn't read the code, but in a more general case these kind of things are rarely found by unit tests. I still doubt that the triggering caused by the file change would have been found in a unit test.
