The plugin brings the excellent CopyParty, which is a file server program. After toggling it on from the Decky plugin, you can go to the provided IP address on any of your devices, enter the password, and access the Deck’s files. You can manage files, upload or download files, and more. File transfer speeds are fast and resumable.
Here’s the GitHub for the plugin, although for right now it’s pretty bare on a description. For details one what all it can do, I’d recommend checking out the CopyParty link up above.
This is not related to the solution posted, but may be useful to someone:
My NAS has been amazing for this. This obviously needs additional hardware, but can be done by connecting a USB stick to your existing router. Works great as the middle man to move files to any device (PC, Steam Deck, handheld emulation console, etc). Also works great to auto sync the files with syncthing.