title: How to validate input file name in laravel 5.6
body: <p>I want to validate image name in laravel. How can I get the value of input image name and pass it to <code>rules()</code> method in custom created <code>UploadRequest</code>. I want to validate if image name is maximum 20 characters and only alpha numeric.</p>
  
  <p>uploadRequest file:
  <img src="https://i.stack.imgur.com/HKH3I.png" alt="uploadRequest file"></p>
  
  <p>upload form view:
  <img src="https://i.stack.imgur.com/zmFRH.png" alt="upload form view"></p>