PHP and AJAX file upload: $_FILES array empty

I'm trying to build an application which allows for file uploads in PHP. When the file is uploaded, a download link should be appended to a specific div. I am using an ajax call to run the php script so that I can have the file download link echoed without getting stuck on the php page. (I hope that makes sense.)

Here is the html:

<form action="php/upload.php" method="POST" enctype="multipart/form-data" id="uploadform">
                Select file to upload:
                <input type="file" name="fileToUpload" id="file">
                <input id="create" type="submit" value="Upload File">
</form>

Here is the external javascript/jquery that makes the ajax call:

$(function () {
  $('form').on('submit', function (e) {
          //prevents reloading of page
          e.preventDefault();
          //get file data
          var file_data = $("#file").prop("files")[0];
          var form_data = new FormData();
          form_data.append('file', file_data);
          //run upload.php script and append result to draggable block
          $.ajax({
            type: 'post',
            url: 'php/upload.php',
            cache: false,
            data: form_data,
            success: function (data) {
              $( "#draggable"+i).append(data);
              i++;
            }
          });

        });
});

And here is the external php script that is supposed to upload the file:

<?php

    $target_path = "../uploads/";
    $target_path = $target_path . basename( $_FILES[ 'fileToUpload' ][ 'name' ] );

    if ( move_uploaded_file( $_FILES[ 'fileToUpload' ][ 'tmp_name' ] , $target_path ) ){
        //echo '<a href=' . $target_path . ' download>Download</a>';
    } 
    else {
        echo $target_path;
        //var_dump($_FILES);
    }

I get the following error: Uncaught TypeError: Illegal invocation

The problem came from trying to pass 'form_data' in the ajax call. I am trying to do that because the $_FILES array was coming up empty once the php script was ran. I need a way to make sure the file that I'm uploading gets stored in that array.