I think one of the thing that surprised me the most when switching from WordPress to Ghost is the lack of file uploader. You can only upload images in Ghost. Even PDF upload is not possible and don't even think of zip files.
Of course you can do the old school way by using SCP command in terminal but most of the time it's not really practical. You have files all over the place and the file name is not exactly helpful.
FileZilla is totally free and can handle login using SSH keys. Just define an SFTP connection and add your keys - FileZilla will handle the rest. Then you can see the directory on both your local machine and the server your Ghost installation resides on.
Go to /var/www/ghost/content and create a folder there or you can just dump into the exisiting images folder. Upload your files there and point the link to yoursite.com/content/<folder & file name>