Make sure "Make computer visible" is checked in your preferences. Click the Receive Files button from the Preferences window, and it's a matter of enabling "Receive files in Downloads folder over Bluetooth" to allow paired devices to send files to your Ubuntu computer. Simple. Just the way it should be.
You might want to disable notifications though, especially when receiving large numbers of files to prevent a glut of notification pop ups. It gets annoying closing all those windows.
Simple (but clunky) Bluetooth on Ubuntu