Views suggestion preprocessor template.php and views-view-field--suggestion.tpl.php working in harmony?
I think what you are looking for is:

function MY_THEME_views_view_field__frontpage_fold__page__field_video_background_files_preprocess(&amp;$variables, $hook) {


Note the addition of _preprocess at the end of your function name.

https://api.drupal.org/api/drupal/modules!system!theme.api.php/function/hook_preprocess/7