I found USB cable file transfer to be somewhat "temperamental", but I can get it to work, more often than not, like this:
Connect two devices using USB cable.
On Mobile select the Settings - Connected Devices - USB
On Desktop, locate the external device icon, but don'yt"open" it quite yet.
On Mobile, select the "File transfer "radio-button", activate.
After a couple of ceconds, if nothing happens on the desktop, the radio-button will revert from File transfer back to No data transfer. However, if the device icon is clicked on the desktop before that, the file navigation window will open with "Internal Shared Storage" icon that represents the root of the filesystem on the mobile.
(This might or might not work for you...)