File upload extension validation issue

I am working in a project Laravel 5.4. When I upload a file(doc,docx,pdf) it uploaded successfully. But If I implement validation in controller like

$this->validate(request(), [
        'user_name'        => 'required',
        'attachment'       => 'required|mimes:pdf,doc,docx',
    ]); 

it redirect to the input form with The attachment must be a file of type: pdf, doc, docx.

I tried with doc and pdf file it returns same. Where is my problem thank you.

via Chebli Mohamed