arma-server-web-admin/public/js/tpl/missions/upload.html

<form class="form" role="form" enctype="multipart/form-data">
  <div class="form-group">
    <label for="mission" class="control-label">Mission File</label>
    <input type="file" class="form-control path" name="mission" id="mission" data-field="mission">
    <span class="help-block">Only supports missions packed as a PBO</span>
  </div>
  <button type="submit" class="btn btn-primary ladda-button" data-style="expand-left">
    <span class="glyphicon glyphicon-cloud-upload"></span>
    Upload
  </button>
</form>