The createObjectURL() creates references to the files stored in the memory. It is useless to send these references to the background script, because when the panel unloads, the references cease to exist.
The "close" button is by the "open tabs", "minimize", "windowed" and "synchronization" buttons, as well as any nearby extension icons.
There's nothing more frustrating than misclicking it and losing an entire window full of tabs, all because Opera's settings don't include an opt-in prompt for confirming whether or not someone really intends to nuke their current window.