Even if you got that to work, there’s another problem I think you’re going to run into. It’s not possible to read or write the u-blox flash file system without modifying system firmware, because the accessor functions are not exported. So even if you could download or upload by FTP, there wouldn’t be a way to access the data.
It is possible to use FTP from user firmware, however. This is upload. Download by FTP doesn't really make sense; HTTP would be a much better choice for download.