There are probably lots of ways to do what you want but here is what I
would do if somebody asked me to program the middle JPanel.

1)  Override paintComponent() in the JPanel to draw a series of images
of your moving file.

2)  Implement Runnable and in the run() create a loop with a delay that
increments a state variable that the paintComponent() can use to redraw
the moving file image.  After each delay, call repaint() to draw the new
position of the file image.

3)  When you start your download, create a new Thread with the Runnable
from the middle JPanel and start it.  When you are done downloading the
file, stop the thread.  The easiest way to do that is to trap the
InterruptedException from the Thread.sleep() delay and exit the run().

Pseudo code:

class FilePanel extends JPanel implements Runnable {
    static final int NUMBER_OF_STATES = 8;
    private volatile int state;

    public void run() {
        while (true) {
            ++state;
            state %= NUMBER_OF_STATES;
            try {
                Thread.sleep(100);
            } catch (InterruptedException ie) {
                break;
            }
        }
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        switch (state) {
            case 1: // paint the file on the left
                break;
            case 2: // paint the file 1/4 of the way
                break;
            ...
        }
    }