Android - File transfer over USB cable option no longer appears after upgrade to MIUI 10

Though I was pretty sure I'd transferred files with at least one of the USB cables I tried around the time I posted this question, I tried a third cable at work tonight and the file transfer option now appeared!

So unless it was a coincidence, it seems it was due to a USB cable that was designed only for charging and lacked the wiring for file transfers!

On Windows PCs and other desktop PCs:

• try to use a different USB port, often rear ports have more power than the front ports which is sometimes needed by some devices
• use an USB 2.0 port if your mainboard supports both ports next to a possible type c port.
• make sure ADB and your device drivers are installed (you can find your device drivers on your phone manufacturers homepage.

On your mobile device:

• go to settings -> "my device" (or similar) and tap on your build number multiple times
• go to "advanced" -> "developer settings" and check if "Select USB Configuration:" is set to MTP for file transfer or PTP for your Android to play a digital camera storage.