I provide here two very acceptable workarounds. Trying to make mtp work for my N7 seriously broke my OS.
Through USB with adb (command line)
adb is android debug tool. It lets you access your android device through command line. You don't need root (though it doesn't hurt).
In order to get access to the developper options, go in 'settings > about tablet' and tap 7 times on the 'build number' field. Then come back to the 'settings' and you will have a new 'developper options' field. Go in there and enable 'USB debugging'.
From your terminal :
adb push ~/Documents/example /sdcard/Ubuntufiles/
Will move a file called 'example' from your Document folder in your Ubuntu Home to the folder 'Ubuntufiles' on your N7's 'sdcard'.
Using the following command from Ubuntu :
you get a remote terminal to your N7, where you can do stuff the GNU way.
This might be off-topic, but if your aim is to move files between your computer and your N7, then the USB prerequisite might not be needed. As long as your N7 and your computer are connected to the same router.
- Download an app that creates a FTP server on your N7.
People seem to love 'software data cable', but it really sends you a barrage of apps. 'File expert' is a file browser that doubles as FTP server, SSH server and plenty of other cool stuff. So that's what I use.
- Activate the FTP server on your N7.
On 'software data cable', it's a huge 'start service' button (brace yourself for fullscreen ads). On 'file expert' : My tools > Share content > Share via FTP
It will display you the IP address and the connection creds you need to feed to Ubuntu.
- Connect to N7 from Nautilus on Ubuntu
Nautilus is your file browser.
You need only to do that once. In the future, you'll have a shortcut in Nautilus.
File > Connect to server. A little window shows up that asks you for the very information that your android app is displaying. Input them and hit 'Connect'.
There you are. A file browser window with the content of your N7. Move files around to your heart's content.