<!-- File input for data upload (hidden). -->
<input class="form-control" type="file" id="data_upload" accept="text/csv,text/comma-separated-values,text/plain,.csv">
<!-- Custom button to trigger file selector for data upload (visible). -->
<label class="form-label" for="data-upload-select">Upload a CSV file</label>
<div class="input-group">
  <button id="data-upload-select" type="button" class="btn btn-outline-primary btn-sm">
    <span class="glyphicon glyphicon-file"></span> Select file
  </button>
  <input type="text" id="data-upload-name" class="form-control" placeholder="No file selected" disabled>
</div>
<!-- Warning text. -->
<small id="data_upload_warn" class="form-text text-primary shiny-html-output"></small>
