Folks (welcome to the distant future of 2024)
I'm pretty sure that there is no sensible way to provide a bulk Azure Blob upload facility in a web hosted app (Wasm, JS or whatever). There could be thousands of files with a total size up to hundreds of MB. Managers are currently using a WPF program I created for uploads and it feels like a perfectly natural process on the desktop, and it's pretty fast using streams on multi cores (I also optionally check for new or changed so only diffs are uploaded which often saves a lot of time).
None of the components or controls I've seen are designed for huge uploads, and in any case, I've reported that it's technically and usability questionable to have "normal" users of the browser app doing this sort of thing. The boss of the app suite is now considering the bigger picture and the bulk upload feature may be delayed or moved to somewhere else in the flow, or the desktop program will suffice. So I'm happy the issue is on-hold for now.
I think this is a good example of how the web browser should never have evolved into a host for business apps. I think the web browser, HTML, HTTP, REST, css and JS have diseased 21st century IT.
Greg K