Use this code to add HTML directly in your input field. This example adds HTML5 placeholder text and required attributes. This can also be done in your customizable HTML by changing [input] to [input required="required"] or [input placeholder="2"]. Any other HTML can be added in this code or to your customizable HTML as well.

add_action('frm_field_input_html', 'add_input_html');
function add_input_html($field, $echo=true){
  $html = '';
  if($field['id'] == 25){ //change 25 to the ID of your field
     $html = ' required="required"';
  }
  if($field['type'] == 'number'){
     $html = ' placeholder="2"';
  }
  if($echo)
    echo $html;
  return $html;
}

If you would like your input fields to show the field title on hover, you can add it in your customizable HTML with [input title="The field name"]. If you would like this to be on a larger number of fields, php may be easier.

add_action('frm_field_input_html', 'add_input_title');
function add_input_title( $field, $echo = true ) {
  $html = '';
  if($field['type'] == 'text'){
     $html = ' title="'. esc_attr( $field['name'] ) .'"';
  }

  if($echo){
    echo $html;
  }

  return $html;
}

Use this code example to disable file URLs for an uploaded file even when file protection is turned off.

add_action( 'frm_field_input_html', 'disable_dropzone_urls' );
function disable_dropzone_urls( $field ) {
	$field_id = 4626; // change 4626 to your field ID
	if ( $field_id !== (int) $field['id'] ) {
		return;
	}

	global $frm_vars;

	$key = 'file' . $field_id;
	if ( ! array_key_exists( $key, $frm_vars['dropzone_loaded'] ) ) {
		return;
	}

	array_walk(
		$frm_vars['dropzone_loaded'][ $key ]['mockFiles'],
		function( &$mock_file ) {
			$mock_file['accessible'] = false;
		}
	);
}