[NiLuJe]

What you're left with is risking filesystem corruption by hard-rebooting the device after transferring the file, after making sure FS/HW caches are flushed, but *before* ejecting it, like we did in the olden days of forced downgrades.

(Since, to make things simple, that file is rejected by a part of the system that's dormant while the device is in mass storage mode; but it will be accepted by the low-level updater at boot time, before said bit of system will ever see it agin. But if you eject, system wakes up, is angry at your implied trickery, and shouts at you.).