The easiest and fastest way would be to create a shared folder:
With the "shared folders" feature of VirtualBox, you can access files of your host system from within the guest system. This is similar how you would use network shares in Windows networks -- except that shared folders do not need require networking, only the Guest Additions.
Shared folders must physically reside on the host and are then shared with the guest
To share a host folder with a virtual machine in VirtualBox, you must specify the path of that folder and choose for it a "share name" that the guest can use to access it. Hence, first create the shared folder on the host; then, within the guest, connect to it.
In the window of a running VM, you can select "Shared folders" from the "Devices" menu, or click on the folder icon on the status bar in the bottom right corner.
Just follow the VirtualBox documentation.
It's the fastest way because it doesn't require any "real" network service in between (like FTP, as you mentioned), or even using Dropbox (which is too slow for that purpose, albeit easier to set up).