title: Restrict file upload to some file extensions
body: <p>I am having a problem with uploading files. I want to allow users to upload files that the system allows...</p>
  
  <p>For example, I allow files having an extension of <strong>*.jpg</strong> to be uploaded by users. So, in the file selection window they must see only files with the <strong>jpg</strong> extension.</p>
  
  <p>How can I get this in RoR?</p>