gistfile1.txt

$args = array(
  					'post_type' => 'attachment',
						'meta_query' => array(
							array(
								'key' => 'ninja_forms_field_id',
								'value' => $field_id,
								'type' => 'numeric',
								'compare' => '='
							),
							array(
								'key' => 'ninja_forms_file_key',
								'value' => $key,
								'compare' => '='
							)
						)
					);
					$query = new WP_Query( $args );