Render Drupal fields inside the rendered markup of other Drupal fields.

gistfile1.php

<?php

/**
 * Implement hook_file_view_alter().
 * 
 * @param type $build
 * @param type $entity_type
 */
function MODULE_file_view_alter(&$build, $entity_type) {
  $qp = QueryPath::withHTML($build['field_caption']['0']['#markup']);

  // Captures the contents of the last element of the caption. If there are multiple
  // paragraphs, this is the last one.
  $html = $qp->find(':root body')->children(':last-child')->innerHTML();

  // Append rendered author and publisher field.
  $html .= render($build['field_author']);
  $html .= render($build['field_publisher']);

  // Replace the contents of the last element.
  $qp->find(':root body')->children(':last-child')->html($html);

  // Set the render markup to the new value.
  $build['field_caption']['0']['#markup'] = $qp->find(':root body')->innerHTML();
}