Cant Upload single image

not sure what is happening with my code but i was trying to upload poster image for articles

// in blade 
<div class="card-footer">
    <button type="button" class="btn btn-icon fuse-ripple-ready" aria-label="add person">
        <i class="icon icon-folder-multiple-image"></i>
    </button>
    <input type="file" name="poster" value="">
</div>

the form will post data to controller function

// linke this
<form action="{{route('articleContentStore', $article->id)}}" method="post" enctype="multipart/form-data">
{{csrf_field()}}

in my controller i have this method

    public function articleContentStore(Request $request, $id)
    {

        $request->validate([
            'poster' => 'nullable|image|mimes:jpg,png,jpeg,gif|max:5000',
            'hide_poster' => 'int:1,0',
            'short_description' => 'nullable|string',
            'hide_introduction' => 'int:1,0',
            'connector' => 'required|string',
            'hide_connector' => 'int:1,0',
            'description' => 'nullable|string',
            'hide_description' => 'int:1,0',
        ]);

        $updateContent = Article::findOrFail($id);

        $updateContent->poster = $request->input('poster');
        $updateContent->thumbnail = $request->input('thumbnail');

        $updateContent->hide_poster = $request->input('hide_poster') ? : 0;
        $updateContent->short_description = $request->input('short_description');
        $updateContent->hide_introduction = $request->input('hide_introduction') ? : 0;
        $updateContent->connector = $request->input('connector');
        $updateContent->hide_connector = $request->input('hide_connector') ? : 0;
        $updateContent->description = $request->input('description');
        $updateContent->hide_description = $request->input('hide_description') ? : 0;

        $updateContent->save();
        
        flash('success');
        
        return redirect()->route('articleContent', compact('id'));

    }

The post is returning correct data but im getting Null value for poster

return $request;  // this is returning bellow
{"_token":"CV8zxIji3HsO3hnD3G4VCtaTeJmcpQo6gGdu8oqT","short_description":"this is the introduction to of the article","hide_introduction":"1","connector":"Read more","hide_connector":"1","description":"this is article full content","hide_description":"1","poster":{}}

poster is empty

Any idea on what is happening