You probably loaded your files in MSC mode, and the firmware update switched the mode back to MTP. Files loaded in one mode are invisible to your PC when connected in the other mode. Switch to MSC (Settings>USB Mode) and check again for your files.
The clip plays them fine, but the PC does not see anything in any USB mode…
Frits
Make sure your Clip is indeed in MSC mode. I had mine flip back to Auto even though it was switched to MSC. It did this to me a couple of times but the setting now seems to have stayed in MSC mode as I set it after connections to my PC.
After the firmware update, the clip will be reset to Automatic mode and the radio pre-sets are lost. There may have been other things. Immediately after the update, bring up settings and check everything. No other problems on my unit and if this update does anything to the operation of the unit I don’t see it.
I’ve long believed updates on anything are usually not worthwhile unless to fix a problem you are experiencing, however, most times I don’t follow my own advice just to confirm my beliefs.