• Download X-Plorer directly on Mi Box (no sideloading required)
• Start it
• Navigate to "WiFi Server" on the right panel and enable it
• On your PC, in a browser window type the address and port that's displayed in X-Plorer

That's the simplest for PC access: you download directly from the box, or upload directly from PC, no intermediate steps required.

However it also supports Cloud Storage services like Dropbox and Gdrive if you already have files there

File commander has a really simple built in on demand web upload client.

You open the app and start the service and then browse on your phone and click upload, select the file and bam it's there.

Try SHAREit app.

Get a USB 2 or 4 way hub and a USB stick copy data to the USB stick and plug it into the mi box

Is the storage adopted storage where it'll only work on the device that set it up, or still independent where you can plug it into any device and read it without needing the encryption key or anything? If the former an FTP server should work fine. If the latter, I'm not sure as I never really tried that

I usually use dropbox or gdrive. Upload from PC. Download with an app on the Mi Box. ES File Explorer IIRC but I might have switched to something else.

I currently have Join installed on my MI Box, and so I can send files directly through the app, from either my phone or my desktop. When I send an apk it automatically opens and prompts to install which is sweet.

Run an FTP server on the MiBox.