I have tried to use the above robot there is a need to add a delay :( also you cannot debug or do something else because you lose the focus :(
//open upload window upload.click();
//put path to your image in a clipboard
StringSelection ss = new StringSelection(file.getAbsoluteFile());
Toolkit.getDefaultToolkit().getSystemClipboard().setContents(ss, null);
//imitate mouse events like ENTER, CTRL+C, CTRL+V
Robot robot = new Robot();
robot.delay(250);
robot.keyPress(KeyEvent.VK_ENTER);
robot.keyRelease(KeyEvent.VK_ENTER);
robot.keyPress(KeyEvent.VK_CONTROL);
robot.keyPress(KeyEvent.VK_V);
robot.keyRelease(KeyEvent.VK_V);
robot.keyRelease(KeyEvent.VK_CONTROL);
robot.keyPress(KeyEvent.VK_ENTER);
robot.delay(50);
robot.keyRelease(KeyEvent.VK_ENTER);