This happened to me too after I the installed wrong firmware. Windows recognized the device as adfu but all future attempts to install any firmware, including firmware that had successfully installed before, were met with 'invalid upgrade firmware file'.
I thought I'd killed mine as it was still shown as adfu device but managed to get it back to upgadeable state by dragging the tweezers down both sides of the chip whilst connected to the pc. I heard the ding-dong noise of the device being unloaded by windows. I was then able to do the trick of bridging contacts on just the one side of the chip whilst connected and windows recognized the adfu device again and I could install firmware again.
Still not found the right firmware though so my player is still just a usb flashdisk
ops: