I was using this method for months untill Makinavaja lets me him obsonet.
The only problem could that you need Linux on the PC where MSX is connected to transfer files from PC to MSX, is a windows problem.
The tech is use Internestor Lite on MSX, when the msx is connected you start the tftp server on your msx, and then you can send all the files you like from any pc on your network to your MSX.
Too criptic?
The best thing in this method is that you have your msx connected to internet
The other way (easy) is use terminal programs on both computers, erix in msx, and use xmodem ymodem or zmodem to transfer the files.
greets