Upload the file using Robot class

Robot robot = new Robot();
StringSelection filepath = new StringSelection(""file path"");
Toolkit.getDefaultToolkit().getSystemClipboard().setContents(filepath,null);
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.keyRelease(KeyEvent.VK_ENTER);