File upload in PHP giving error

I am building an web app using PHP.I am creating a file upload section in it but i am having problem.

<form action="upload.php" method="post" enctype="multipart/form-data">

Select image to upload:

<input type="file" name="fileToUpload" id="fileToUpload">

<input type="submit" value="Upload Image" name="submit">

</form>

upload.php:

<?php

$target_dir = "images/";

$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);

$uploadOk = 1;

$imageFileType = pathinfo($target_file,PATHINFO_EXTENSION);

// Check if image file is a actual image or fake image

if(isset($_POST["submit"])) {

$check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);

if($check !== false) {

    echo "File is an image - " . $check["mime"] . ".";

    $uploadOk = 1;

} else {

    echo "File is not an image.";

    $uploadOk = 0;

}

}

// Check if file already exists

if (file_exists($target_file)) {

echo "Sorry, file already exists.";

$uploadOk = 0;

}

// Check file size

if ($_FILES["fileToUpload"]["size"] > 500000) {

echo "Sorry, your file is too large.";

$uploadOk = 0;

}

// Allow certain file formats

 if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType !=    "jpeg"

&& $imageFileType != "gif" ) {

echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";

$uploadOk = 0;

}

// Check if $uploadOk is set to 0 by an error

if ($uploadOk == 0) {

echo "Sorry, your file was not uploaded.";

  // if everything is ok, try to upload file

 } else {

   if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {

    echo "The file ". basename( $_FILES["fileToUpload"]["name"]). " has been    uploaded.";

 } else {

    echo "Sorry, there was an error uploading your file.";

  }

  }

  ?>

It is working fine for images. but when i try to check what happens if i upload mp4 file. it echo File already exists,Sorry, only JPG, JPEG, PNG & GIF files are allowed.Sorry, there was an error uploading your file. Kindly guide me where i am doing mistake so that this work fine for me without having errors. Thanks in advance.