im assuming you are using windows? in the future, mention what you are using, since more than windows users use blender…
in windows, the best way to do it is open up a dos shell, then type ftp [the address] and hit enter. cd to the directory you want to put stuff in, then use a simple put command. im sure you can find tutorials on the web on how to use command line ftp clients. and once you know how to use the command line, you dont have to worry if somebody else has a gui ftp client installed on their machine, or if it uses some bizarre ui…
I like to use FTP explorer. Windows only but very simple and quick with drag and drop. Works just like windows explorer except it’s freeware! (noncommercial, $30 for commercial) It has PASV mode aswell.