I am preparing a macOS app for the App Store, using the App Store Connect site on the web. I have successfully uploaded several screenshots with very fast uploads -- a few seconds -- but uploads of app previews are ending up stuck in processing (the message is "This file has been uploaded and is processing.") for many hours -- e.g., six hours for a 13 MByte preview.
It is also frustrating that if I refresh the web page where I am attempting the upload, the upload fails immediately, and I have to start all over again.
As I write these words I am waiting to see if the current attempt will succeed ...