What most people end up doing is to have 1 or more Storage buckets just for development. This actually works pretty well when you get authentication working (quite easy, more on that later). You have to pay for the traffic you upload and download when testing, but the price is so low that it’s just a rounding error for most usecases. Of course, if your files are very large uploading them could take some time. On the other hand, you probably have to upload them sooner or later anyway. And having them in the real Storage makes them accessible to the rest of the Google Cloud Platform, most of which integrates nicely with Storage.