Besides the option of FTP using Filezilla, you can use USB method as well, as if you were to upgrade the device instead of copying the files from the computer to the device it can be done vicecersa, you can take the following link as a guide of mounting the usb,
my personnal favorite is to go the other way around: from my windows computer,
I open a DOS windows,
ftp to the switch
issue get commands to the files I want.
if i need to, I also issue put commands to put some files on the switch.
don't forget to issue a "binary" command if you try to get compressed files (varlog) requested by JTAC for troubleshooting or the files will be of no use. Also make sure ftp on the switch is system enable. other than that, have fun.
when upgrading images, I'll ftp the whole image on the switches with pre-made bat commands files issued from an Excel files with all the switches ip addresses. and also a couple of .sh file with Junos cli command that I can execute.