hook_file_validate does not work

Good morning,
I try to use mimedetect-module (https://www.drupal.org/project/mimedetect) which offers a way to check if uploaded files mimetype matches the file extension.
To make the validation work, I added

/**
 * Implements hook_file_validate().
 */
function custommedia_file_validate($file) {
  if (module_exists('mimedetect')) {
    $errors = array();
    $detected_mime = mimedetect_mime($file);

    if ($file->filemime != $detected_mime) {
      $extension = pathinfo($file->filename, PATHINFO_EXTENSION);
      $errors[] = t('It was detected the file format was originally (@type) and this does not match the current file format (@extension). Please re-save the image file in the original format and attempt to re-upload.', array('@type' => $detected_mime, '@extension' => $extension));
    }

    return $errors;
  }
}

This works great in all file / media dialogs/widgets besides plupload (in combinaiton with filefield_sources_plupload). If there is a validation error, I get the error message as expected. But, the form UI changes and I have 2 plupload fields now. The first plupload widgets does NOT work afterwards when trying to upload a valid file. The second widget works.

I already checked out some of the issues decribed here in the issue queue - but no suggested solution seems to work.