edit.html.erb

<div class="row">
  <div class="col-lg-12">
    <div class="panel">
      <header class="panel-heading lead">고객 사진 추가</header>
      <div class="panel-body">
        <%= simple_form_for( [:staffs, @user], url: staffs_user_photo_path, html: { class: 'form-horizontal', multipart: true }  ) do |f| %>
            <% if @user.errors.present? %>
                <script type="text/javascript">
                  <% @user.errors.full_messages.each do |msg| %>
                  toastr.error('<%= msg %>');
                  <% end %>
                </script>
            <% end %>
            <div class="form-group">
              <%= f.file_field :files, accept: 'image/*', multiple: true, onChange: 'handleFileSelect()' %>
              <output class="row">
                <div class="col-md-3" id="result"></div>
              </output>
              <%= image_tag '', class: ' avatar_preview' %>
            </div>
            <div class="form-group">
              <div class="col-sm-offset-3 col-sm-6">
                <%= f.submit class: 'btn btn-info' %>
              </div>
            </div>
        <% end %>
      </div>
    </div>
  </div>
</div>

<script>
  function handleFileSelect(e) {
    //Check File API support
    if (window.File && window.FileList && window.FileReader) {
      var files = event.target.files; //FileList object
      var output = document.getElementById("result");
//      var div = document.createElement("div").classList.add("col-md-3");
      for (var i = 0; i < files.length; i++) {
        var file = files[i];
        if (!file.type.match('image')) continue;
        var picReader = new FileReader();
        picReader.addEventListener("load", function (event) {
          var picFile = event.target;
          var aTag = document.createElement("a");
          aTag.setAttribute("href", "#");
          aTag.classList.add("thumbnail");
          aTag.innerHTML = "<img class='thumbnail' src='" + picFile.result + "'" + "title='" + picFile.name + "'/>";
          output.insertBefore(aTag, null);
        });
        //Read the image
        picReader.readAsDataURL(file);
      }
    } else {
      console.log("Your browser does not support File API");
    }
  }
</script>