This should work with Firefox, Chrome and IE drivers.
FirefoxDriver driver = new FirefoxDriver();
driver.get("http://localhost:8080/page");
File file = null;
try {
file = new File(YourClass.class.getClassLoader().getResource("file.txt").toURI());
} catch (URISyntaxException e) {
e.printStackTrace();
}
Assert.assertTrue(file.exists());
WebElement browseButton = driver.findElement(By.id("myfile"));
browseButton.sendKeys(file.getAbsolutePath());