So it doesn't show up as a file; it just causes new programming to be written to the device, which should allow it to reboot as a CircuitPython device with a code.py that you can edit. For that, you may need to use the RESET button (not BOOT) -- after that (fingers crossed), the device should show up as CIRCUITPY instead of UFTHRS2BOOT.
Yes, I do get back the CIRCUITPY and the correct CP version shows in REPL, but the Serial does not work and the code does not run on download when using Mu.