I cannot give you an answer in the near term, as I had to repurpose my raspberry pi for a short term project.
Double check that you're typing "\xxx.xxx.xxx.xxx" into Windows explorer (not internet explorer). Note it's two backslashes ( \ ) before the IP Address. I don't know why it keeps changing it to one.
I'll try to get you an exact solution that works for me, hopefully in the next few days.
Your other option is to mount a USB thumb drive and you can upload everything that way. (This is what I do. It's safer and I have had too many catastrophic failures to risk keeping everything on the SD card.)
You can also FTP into Recalbox as well.
Keep me in the loop in the interim.