Add suggestions for files by media's view mode

Suggestion based on what? As you can see in your output you can use file-video.html.twig. What other suggestion based on what would you need on top?

Ah sorry, it's in the issue title. You want a template suggestion for the file based on the media's view mode.

yes, thank you :)

additionally to file-video.html.twig there should be things such as:

file-media-video.html.twig
file-media-viewmode-video.twig

or similar

We would need to find a way to pass the view mode from the media entity down to the file. Not sure if that's possible easily.

This can be done by passing down the view mode via data attributes from the field template preprocess and then removing the data attribute in the file_video template suggestion hook.

function HOOK_theme_suggestions_file_video_alter(array &$suggestions, array $variables) {

  // Check if there's a view mode available.
  if (!empty($variables['attributes'])) {
    /** @var \Drupal\Core\Template\Attribute $attrs */
    $attrs = $variables['attributes'];
    if ($attrs->hasAttribute('data-view-mode')) {
      /** @var \Drupal\Core\Template\AttributeString $view_mode */
      $view_mode = $attrs->storage()['data-view-mode'];
      $attrs->removeAttribute('data-view-mode');
      // Sanitize the view mode name to use it in the template suggestion.
      $view_mode_sanitized = str_replace('.', '_', $view_mode);
      // Add the new suggestion with the view mode name.
      $suggestions[] = 'file_video__' . $view_mode_sanitized;
    }
  }
}

function HOOK_preprocess_field__file(&$variables) {
  if (
    $variables['element']['#formatter'] == 'file_video'
    && !empty($variables['element']['#view_mode'])
  ) {
    foreach (Element::children($variables['element']) as $key => $value) {
      if ($variables['element'][$key]['#theme'] == 'file_video') {
        $attrs = &$variables['element'][$key]['#attributes'];
        $attrs->setAttribute('data-view-mode', $variables['element']['#view_mode']);
      }
    }
  }
}