Uploading file in Selenium using sendKeys() is not working

Please try the following javascript executor code

String filePath = "D:\\Images\\icons.png";
JavascriptExecutor jsx = (JavascriptExecutor) driver;
jsx.executeScript("document.getElementById('iconFlagFile').value='" + filePath + "';");

From your explanation, I am assuming that, you have an HTML tag that allows you to upload a file from local drive but it is not enabled. Unless you hit submit button it wont allow you to upload a file.

In such case, try to edit the form tag to edit and then try to call sendKeys() method, and you can edit the html elements using javascript executor

Please try this for file upload.

    String filename = "path of the file";
    File file = new File(filename);
    String path = file.getAbsolutePath();
    // give the URL to upload
    driver.findElement(By locator).sendKeys(path);

Use Clipboard class and Robot class combination

    Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();
    StringSelection str = new StringSelection("File Path");
    clipboard.setContents(str, null);

    WebElement element = driver.findElement(By.xpath("elementXpath"));
    element.click();

    Robot robot = new Robot();
    robot.keyPress(KeyEvent.VK_CONTROL);
    robot.keyPress(KeyEvent.VK_V);
    driverwait(1);
    robot.keyRelease(KeyEvent.VK_CONTROL);
    robot.keyRelease(KeyEvent.VK_V);
    driverwait(1);
    robot.keyPress(KeyEvent.VK_ENTER);
    robot.keyRelease(KeyEvent.VK_ENTER);