In order to upload your files, you can either use the Multicraft built-in file manager, or an FTP client such as FileZilla, Cyberduck or WinSCP. If one of your files is larger than 100MB and/or your connection is slow, we strongly advise you to use an FTP client.
When using the Multicraft file manager, you will also be able to see the FTP login details (host, port, username) required by FTP clients. Just log into Multicraft and follow this path:
Files → File Manager