A mutator to get file extension

Hello,

I have a file model like this:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class File extends Model
{
    use HasFactory;
    protected $fillable = [
        'name',
        'file_path'
    ];

    protected $appends = ['extension'];

    public function getExtensionAttribute()
    {
        return \File::extension($this->filename);
    }

}

I tried to get the extensions. But the value is always empty, what I am doing wrong? This is how I store the file, maybe this is important:

$filePath = $request->file('file')->storeAs('assets', $fileName, 'local');