Speaking at least for myself, it's not clear what you actually did. Did you copy the file directly to the B2's hard disk as viewed by NAS? Did you use the Upload facility in the WebUI?
In either case, it is likely that your WiFi signal strength may be playing a part in why the wheel continues to spin for so long.
Check your WiFi signal strength using Settings > Maintenance > WiFi Strength. (I got that from the Brennan website on the Menu page.)
Your signal strength should be at least -50 (-40 being better, -60 being worse) and not fluctuate wildly.
To improve your signal strength, try the following:
Move your B2 closer to your router.
Switch the WiFi dongle to USB C.
Move your WiFi dongle to the end of a USB extension cable.
You can buy a new WiFi dongle that has an antenna. Look for one that has the RT5370 chipset.
If none of that helps, you could try plugging a network cable between the inside back of the B2 and your router (and remove the WiFi dongle).
Another factor that might be playing a part is the folder structure you created for your file. It seems odd (to me) that you're just uploading the one file. I've never used the WebUI to Upload anything. Will it do just a single file, or does it need to receive an album folder, even if that album has only one file?
Have you tried copying the file - in the proper Artist / Album / track(s) format - to a USB stick and Importing it? That might be the simplest thing to do at this point.