Sometimes you may need to run Ubuntu-Mate in a VirtualBox virtual machine, for example at your place of work. One of the critical "settings" in such a situation is the setting up of a "shared-folder" in which you can transfer files between the windows host and the Ubuntu-Mate guest.
I've just been faced with this problem. I consulted the internet. There was above all a lack of clear instructions about how to just get this done, and move on with the rest of your day.
1) In the Windows host
Configure the shared folder to be auto-mounted (it's a setting on the Shared Folder configuration in VirtualBox itself).
2) In the linux client
The regular way of getting access to the files now, is to allow VirtualBox to automount the shared folder (which will make it show up under /media/sf_directory_name) and then to add your regular Ubuntu user to the vboxsf group.
sudo usermod -aG vboxsf
Now you need to reboot the Linux guest.
The vboxsf group has full access. By adding your user to that group, you gain full access. So you wouldn't worry about changing their permissions (which don't make sense on the Windows host), you just give yourself access. This is because the Host OS (Windows) doesn't support the same permission system as Linux, so VirtualBox has no way of knowing who should own the files.
Accessed on 07.09.2016
This worked with Windows-7-Enterprise, Virtualbox 5.1.4 and Ubuntu-Mate 16.04.1 64bit.