Comrades,
Just to clarify this is what worked for me - I pieced the info together from this thread...
http://en.miui.com/thread-858267-1-1.html
Several people hinted at the solution in the thread, but didn't specify exactly how to do it. Here is what worked for me - but this is Android and Windows so it could be some other random setting somewhere in your phone or PC.
I enabled developer options and debugging mode with the usual magic incantation.
I connected my phone and told Windows to open folder when it connects,
On the phone I pulled down the notification screen you will see at the bottom "Android System - USB charging device". You have to use your psychic powers to know that you need to tap this notification.
When you tap it you see "Tap for more options". so...tap for more options.
A new and magic dialogue box opens up that gives a bunch of options. Top of the list, and the default option is "Charge this device". But the second option - which was a winner for me - is "Transfer files". Tap that radio button (while whispering the word "abracadabra" under your breath) and there you go!!! The folder magically appears on my File Manager.
If you think of it like one of those 1980s text adventure games it's actually rather fun.
If you think of it any other way it's a freaking pain in the arse!!!
By the way, if it doesn't work you probably thought I was joking about whispering abracadabra...I wasn't!!
I assume this is a bug that you can't just tell the phone to default to file transfer???
Cheers,
SPuD