If you want Files 3.28 right now you can get the Nightly Flatpak (though that will automatically update you to 3.29 when that’s Flatpak’d).
GET IT HERE ----> https://git.gnome.org/browse/gnome-apps-nightly/plain/nautilus.flatpakref?id=7308f18578ac9d587ef82e1bae8b5e78b52f66aa
Reboot after installing and then when you click on Files in the Dash or Activities it will load up the Flatpak rather than the Deb. Please note that the Files Flatpak is very buggy (and they know that it’s buggy so don’t file bug reports for it). Alternatively you can get the Deb from Debian Experimental (which is highly likely to break your install although it could be less buggy than the Flatpak).
GET IT HERE ---> https://packages.debian.org/experimental/nautilus
You may want to wait for Files 3.28 to hit Debian Unstable or Testing (currently Buster) before getting the Deb from there (and it would still be risky to do so and not recommended). You could thus get 3.28 from Debian but it really isn’t recommended also because you’d get no automatic updates (not even security updates) for it.
Basically, for now, please wait for more progress on the Flatpak and snap!