File uploads error with Unit testing

Hey guys, I am having an issue with my unit testing Can anyone help? Here is my TestController inside tests/Unit

namespace Tests\Unit;

use Tests\TestCase;
use Illuminate\Foundation\Testing\WithFaker;
use Illuminate\Foundation\Testing\RefreshDatabase;
use Illuminate\Support\Facades\Storage;
use Illuminate\Http\UploadedFile;

class ExcelTest extends TestCase
{
    /**
     * A basic test example.
     *
     * @return void
     */
    public function testExample()
    {
        // Fake any disk here
        Storage::fake('local');

        // Create file
        file_put_contents(storage_path().'/app/xls/file.xls', "MC,What is 2+2?,4\n");

        $response = $this->postJson('/excel-demo', [
            'file' => new UploadedFile(storage_path().'/app/xls/file.xls','file.xls', null, null, null, true),
        ])->assertStatus(200);
        /*$response = $this->json('POST', '/excel-demo', [
            'file' => UploadedFile::fake()->create(storage_path().'/app/xls/file.xls')
        ]);*/
        Storage::disk('local')->assertExists('file.xls');

    }
}

Here is my controller ExcelController

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Storage;

class ExcelController extends Controller
{
    public function Excel(Request $request)
    {
        $file = $request->file('file');

        Storage::disk('local')->putFileAs('xls', $file, $file->getClientOriginalName());

        \Excel::load(storage_path().'/app/xls/'.$file->getClientOriginalName(), function($file) {

        })->convert('csv');
        return response([
            'message' => 'uploaded'
        ], 200);
    }
}

I am getting this error while running this test

Runtime:       PHP 7.1.11
Configuration: C:\xampp\htdocs\Application\phpunit.xml

F                                                                   1 / 1 (100%)

Time: 1.43 seconds, Memory: 16.00MB

There was 1 failure:

1) Tests\Unit\ExcelTest::testExample
Expected status code 200 but received 500.
Failed asserting that false is true.

FAILURES!
Tests: 1, Assertions: 1, Failures: 1.```