You have the ability to manage multiple files in any of your Master Vault lockers simply by clicking the "tick box" on the bottom right of each file.

If you hover over the bottom half of the file you will see the tick box appear and you simply click on the box to select it. If you select two files this will also bring up a menu that gives you the options to assign multiple files to a vault or to delete multiple files from a locker.

There is the option to "Select All" on the left menu so that you can select every file in the locker. You still have the ability to de-select a few of the files if you need to so it is very flexible.