Craft CMS plugin example for handling AJAX file uploads

Controller.php

<?php

namespace Craft;

class Plugin_FileController extends BaseController
{
    protected $valid_extensions = array('jpg', 'jpeg', 'png', 'pdf');
    protected $assetSourceId = 1;

    public function actionUpload()
    {
        $this->requireAjaxRequest();

        $errors = array();
        $success = array();

        foreach ($_FILES['files']['error'] as $key => $error)
        {
            if (!$error)
            {
                $filename = $_FILES['files']['name'][$key];
                $file = $_FILES['files']['tmp_name'][$key];
                $extension = end(explode('.', $filename));

                if (!in_array($extension, $this->valid_extensions))
                {
                    $errors[] = "$filename has an invalid extension.";
                    continue;
                }
                
                $uploadDir = craft()->assetSources->getSourceById($this->assetSourceId)->settings['path'];

                if (move_uploaded_file($file, $uploadDir . $filename))
                {
                    IOHelper::deleteFile($file);

                    $file = $uploadDir . $filename;

                    $fileModel = new AssetFileModel();
                    $fileModel->sourceId = $this->assetSourceId;
                    $fileModel->folderId = $this->assetFolderId;
                    $fileModel->filename = IOHelper::getFileName($filename);
                    $fileModel->kind = IOHelper::getFileKind(IOHelper::getExtension($filename));
                    $fileModel->size = filesize($file);
                    $fileModel->dateModified = IOHelper::getLastTimeModified($file);

                    craft()->assets->storeFile($fileModel);

                    $success[] = "$filename was saved.";
                }
                else
                {
                    $errors[] = "$filename was unable to be saved.";
                    continue;
                }
            }
        }

        $this->returnJSON(compact('errors', 'success'));
    }
}

index.html

<form method="post" action="" accept-charset="UTF-8" enctype="multipart/form-data">
      <input type="hidden" name="action" value="plugin/file/upload" />
      <input type="hidden" name="redirect" value="" />

      <p><input type="file" name="file" multiple></p>

  	<input type="submit" class="btn submit" value="Upload">
  </form>

  <div style="margin-top:20px" id="output" />

  {% set js %}
  	$(function() {
  		$("form").submit(function() {
  			var url = '/admin/actions/' + $(this).children("[name=action]").attr('value'),
  				files = $(this).find("[name=file]")[0].files,
  				formdata = new FormData(),
  				file,
  				reader;

  			for (var i = 0; i < files.length; i++) {
  				file = files[i];
  				if (window.FileReader) {
  					reader = new FileReader();
  					reader.readAsDataURL(file);
  				}
  				formdata.append("files[]", file);
  			}

  			$("#output").html("Uploading...");

			$.ajax({
				type: "POST",
				url: url,
				data: formdata,
				processData: false,  
  				contentType: false,
				success: function(json) {
					window.json = json;
					console.log(json);

					var errors = json['errors'],
              success = json['success'],
						  html = "<p><b>" + errors.length + " errors, " + success.length + " successfully uploaded</b></p>";

              $("#output").html(html);
				},
				error: function (xhr, ajaxOptions, thrownError) {
					alert("ERROR: " + xhr.status + " " + thrownError);
				}
			});

		    return false;
  		});
  	})
  {% endset %}

  {% includeJs js %}

@MFFunmaker I tried to implement it according to your changes, however i am running into ERROR: 500 Internal Server Error

this is the form (it throws a syntax error if i change the name to file[]):

  <form class="upload" method="POST" enctype="multipart/form-data">
    <input  type="hidden" name="action" value="assetLinks/upload/UploadFile">
    {{ getCsrfInput() }}
    <input class="file btn" type="file" name="file" multiple>
    <button data-icon="upload" class="uploadfile btn submit" type="submit">Upload file</button>
  </form>

this is the controller:

<?php
namespace Craft;

class AssetLinks_UploadController extends BaseController
{

  protected $valid_extensions = array('jpg', 'jpeg', 'png', 'pdf');
  protected $assetSourceId = 1;
  public function actionUploadFile()
  { 
      $this->requireAjaxRequest();
      $errors = array();
      $success = array();
      foreach ($_FILES['files']['error'] as $key => $error)
      {
          if (!$error)
          {
              $filename = $_FILES['files']['name'][$key];
              $file = $_FILES['files']['tmp_name'][$key];
              $filenamePieces = explode('.', $filename);
              $extension = end($filenamePieces);
              if (!in_array($extension, $this->valid_extensions))
              {
                  $errors[] = "$filename has an invalid extension.";
                  continue;
              }

              $uploadDir = craft()->assetSources->getSourceById($this->uploadAssetSourceId)->settings['path'];
              $uploadDir = str_replace('{basePath}', craft()->config->get('environmentVariables')['basePath'], $uploadDir);
              if (move_uploaded_file($file, $uploadDir . $filename))
              {
                  IOHelper::deleteFile($file);
                  $file = $uploadDir . $filename;
                  $fileModel = new AssetFileModel();
                  $fileModel->sourceId = $this->assetSourceId;
                  $fileModel->folderId = $this->assetFolderId;
                  $fileModel->filename = IOHelper::getFileName($filename);
                  $fileModel->kind = IOHelper::getFileKind(IOHelper::getExtension($filename));
                  $fileModel->size = filesize($file);
                  $fileModel->dateModified = IOHelper::getLastTimeModified($file);
                  craft()->assets->storeFile($fileModel);
                  $success[] = "$filename was saved.";
              }
              else
              {
                  $errors[] = "$filename was unable to be saved.";
                  continue;
              }
          }
      }
      $this->returnJSON(compact('errors', 'success'));
  }
}

Hello,

$this->requireAjaxRequest();

This is defined any where or It is Craft by default function. How I can use in Craft 3. Can you please guide me for this?