How to preview uploaded image instantly with paperclip in ruby on rails

If I understand you correctly you want to preview an image before it is actually uploaded, right? You can do so by using some javascript.

By setting the width and height attributes of the image tag you can simulate a thumbnail.

$(function() {
  $('#pictureInput').on('change', function(event) {
    var files = event.target.files;
    var image = files[0]
    var reader = new FileReader();
    reader.onload = function(file) {
      var img = new Image();
      console.log(file);
      img.src = file.target.result;
      $('#target').html(img);
    }
    reader.readAsDataURL(image);
    console.log(files);
  });
});

and the html:

<form>
  <input type="file" id="pictureInput"> 
</form>
<div id="target">
</div>

you can check it out on code pen