Another way to transfer files - in my use case because I'm using an IDE in the host but compiling code on the ARM QEMU guest - is to setup something like
fswatch in combination with a file syncing or copying tool (scp or rsync). When correctly configured this will copy files to the QEMU machine whenever they change.
scp on its own should be sufficient to move files to or from the guest without resorting to the risks of a mount-unmount cycle but requires running manually.
Couple of assumptions I've made: your host is a modern OS. fswatch works cross-platform but in subtly different ways. I've also assumed that the guest is running an ssh daemon. There's nothing stopping fswatch using FTP (and a corresponding guest FTP daemon) if that works for you. A Linux host also has inotify-based tools.
It's not strictly a shared folder but the end result is mostly the same.
HTH.