Uploading a file - Getting null from request

I'm trying to make a fileupload but the request is returning null and i don't know why... Can anyone help me?

Here my routes file:

Route::get('/materias', 'MateriasController@index');
Route::get('/materias/novo', 'MateriasController@create');
Route::post('/materias', 'MateriasController@store');

My view file:

<form action="/materias" method="post">
                {!! csrf_field() !!}
                <div class="form-group">
                    <label>Nome da matéria:</label>
                    <input class="form-control" type="text" name="name">
                </div>
                <div class="form-group">
                    <label>Imagem banner:</label>
                    <input type="file" name="file">
                </div>
                <div class="form-group">
                    <button type="submit" class="btn btn-primary">Criar matéria</button>
                </div>
 </form>

And my controller:

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Http\Requests;

/**
* the upload method
*/
public function store(Request $request)
{
        if ($request->hasFile('file'))
        {
            return 'Uploaded';
        }
}

Really, any help will be apreciated guys :D