discourse-custom-wizard/assets/javascripts/wizard/templates/components/wizard-field-upload.hbs

<label class="wizard-btn wizard-btn-upload {{if uploading 'disabled'}}">
  {{#if uploading}}
    {{i18n "wizard.uploading"}}
  {{else}}
    {{i18n "wizard.upload"}}
    {{d-icon "picture-o"}}
  {{/if}}

  <input disabled={{uploading}} type="file" accept="image/*,application/pdf" style="visibility: hidden; position: absolute;" />
</label>