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="missions" class="control-label">Mission Files</label>
    <input multiple="multiple"
           accept=".pbo"
           type="file"
           class="form-control path"
           name="missions"
           id="missions"
    >
    <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>