Array to string convertion, Laravel Validation

I have form where i have an input to do a file upload:

HTML:

<form method="POST" enctype="multipart/form-data" action="/admin/dashboard/category/{{$action}}">
...
    <fieldset class="form-group">
        <label for="image">Imagem</label>
        <input type="file" id="image" name="mainImg">
    </fieldset>
...
</form>

ControllerPost:

use Validator;

....

$rules = array('mainImg' => 'image|max:1024*1024');
$messages = array(
    'mainImg.image' => 'Must be an image (jpg, gif or png)',
    'mainImg.max' => 'heavy file... upload max is 1 MB'
);

$validator = Validator::make($request->all(), $rules, $messages);

if($validator->fails()) {
    return redirect()->back()->widthErrors($validator);
}
else {
    dd('heya');
}

When i sumbmit the form this message appears:

ErrorException in FileLoader.php line 109: Array to string conversion

Someone knows why, and how do i solve it?

via Chebli Mohamed