无法使用PHP和AJAX上传文件

Question

我对AJAX很新，我遇到了无法将文件上传到所需文件夹的问题。上传似乎很好，但文件没有出现在F:/xampp/htdocs/pgadmsn/uploads/中。也许我错过了一分钟，请帮助我。

uf.html：

<form action="upload.php" method="post" enctype="multipart/form-data">
    <label>Select a File to Upload</label> <input id="fileToUpload1" type="file" name="fileToUpload1" /> 
    <input type="button" value="Upload" onclick="loadFile()" />
</form>
<script>
function loadFile() {
    var myFileList = document.getElementById('fileToUpload1').files;
    var fileToUpload = myFileList[0];
    alert(fileToUpload.name);
    var xhr = new XMLHttpRequest();
    xhr.open("POST", 'http://10.192.44.143/pgadmsn/upload.php');
    xhr.send(fileToUpload);
}
</script>

upload.php的：

<?php
$target_dir = "F:/xampp/htdocs/pgadmsn/uploads/";
$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.";
    }
}
?>