<input type="file" ngf-select ng-model="vm.attachment" name="file" accept="application/pdf" ngf-max-size="5MB" ngf-model-invalid="errorFile">

<md-button id="uploadButton" class="md-raised md-primary"> Choose Files </md-button>

link (scope, element, attrs) {
  const input   = element.find('#fileInput');
  const button = element.find('#uploadButton');

  if (input.length && button.length) {
    button.click((e) => input.click());
  }
}

<button ngf-select ng-model="picFile" accept="image/*"> Select Picture</button>
File name: {{picFile.name}}